First, you must ZERO your level. This is a zeroing, NOT a calibration. A calibration would be to set the sensitivity to a certain number. That number would be an angular amount or amount of linear difference between the two sides that is indicated by the distance between the lines on the vial. I have never seen a vial level that can be calibrated: perhaps electronic ones can but I do not have any experience with them. All levels can be ZEROED.
Next, you said a "machinists level". There are different machinist levels and they have different sensitivities. The most sensitive ones will show a stated number of ten thousandths they will detect over a distance of one foot (or some say per ten inches). Others are only able to show thousandths over that distance. To really level a lathe, you will need the style that shows ten thousandths. You can get it close with a thousandth reading level but you will need to do more additional work to be sure the bed is not twisted. It would be helpful in this discussion to know the sensitivity of your level.
PREPARE THE BENCH & LATHE:
The next thing is what type of floor is your lathe bench sitting on? If it is a wood floor, you can stop now as you will never achieve tenths with a lathe on a wood floor. It will change more than that just when you shift your position on a stool next to the lathe bench (been there, done that, it really happens). Then, what type of bench is it on? A metal bench is best. I have my SB-9 on a welded steel bench. A very well built wood bench could work, but wood changes over time and you would need to re-level the lathe more often.
Next, your floor is not going to be flat even to a thousandth so your bench MUST sit on the exact, same spots on the floor at all times. The best way to do this is to bolt it to the floor. The feet should have levelers so the bench can be nominally level before you start with the lathe itself. This saves a lot of time and shims.
I would then bolt ONE end of the lathe down with fairly tight bolts and the other end just finger tight. This allows the bed to assume it's own preferred position. Only after getting to this place in the overall procedure can you even try to ZERO your level. The SB-9 has a flat way on the bed and this is probably the best place to zero your level, not across the ways. Remove the tail stock and do a good job of cleaning this flat way. If tenths count, then you will need to place the level on the EXACT same spot on this way so you should use something, like the carriage, to set the level against. Your level should have a second vial to show that it is roughly level in the cross direction. Use that vial while reversing the level to ensure that the lathe bed is approximately level from front to back while the level is in position on the flat way. If needed, add some shims under the front or rear feet of the lathe to achieve this.
ZERO THE LEVEL:
Finally you have a repeatable surface to sit it on and are ready to ZERO your level. Place the level on the flat way and with one end against the carriage. Observe the position of the bubble. If the bubble is off one end, you will need to add some shims under one end of the lathe to bring it into sight. You are not trying to actually level the lathe yet, just be able to read the bubble. Remember the position of the bubble and reverse the level end-for-end. Again, set it against the carriage so that it sits on the same position on the flat way. Now read the bubble again.
Adjust your level until the bubble is in the same position for both ways the level is sitting on the way. This means that it will be the same distance from the level's center and on the same side of that center in relation to the lathe and the room. But it will be on the opposite side of the level as the level is reversed. When this condition is met, your level is properly ZEROED. Now you can use it to level the lathe.
LEVELING THE LATHE
At this point in time a SB-9 IS a used machine. This means that the rather small, flat tops on the ways may and VERY PROBABLY WILL have some dings in them. Even when this lathe was new, I know of no source that says that they were scraped flat and parallel to the other seven surfaces of the lathe ways. For this reason I have never trusted a level placed on top of those small flats. If you insist on using them, I would strongly recommend stoning them with a ground flat stone first. I would set the stone across the three vee ways and stone all three of them at once. But with my SB-9 I simply used the carriage to ride on the vee ways and placed the level on top of the carriage with the compound removed. Again, the SB-9 is an older machine and the castings have had to warp so check the seating of the level and stone the top surface of the carriage as needed. Mine was so bad that I had to actually scrape it first and then stone it. Another consideration here is the wear on the vee ways. Mine are noticeably worn so the results can not be the same as with a freshly ground and/or scraped set of ways.
Since your level is ZEROED, you can now just place it on and leave it in one place on the top of the carriage. You can carefully move the carriage from one end of the lathe bed to the other and observe the bubble. Start with the head stock end tightened down and the tail stock just finger tight as described above. See which way the bed twists. Add a shim as needed at the tail stock end and repeat the test with all four bolts medium tight. Keep adding shims until you achieve the best level and with all four bolts tight.
FINAL TUNE:
That is as good as you can do with a level but you are not finished. So don't obsess about that last tenth or two. Now the fine tuning starts by using other methods involving measuring on and turning test bars.
