You have an external thread with a known pitch (24 TPI) and an approximate OD (1.39"). You may have measured the actual OD to ten decimal places, but it is still only APPROXIMATE as it relates to the theoretical OD of your actual thread which may be larger or smaller (most likely larger).
Threads may be specified by the OD, but measuring the OD is NOT a dependable way to ensure a fit. The problem is most threads have truncated peaks and there is no good way to measure or estimate the actual percentage of this truncation. For a standard thread form it is a nominal 1/8th of the height of the sharp edged thread form BUT this is only a nominal value and even the standard allows for a large variation. There are many reasons which may cause this number to be different from that nominal value. And, as I said, there is no good way to directly measure it.
This is why we have thread wires and thread micrometers. They measure the pitch diameter of the thread and that is the real basis of the calculation that you need to do. I suggest that you read the procedure and the math for the three wire method and use it to find the pitch diameter. Then use the standard thread formulae to calculate the ID you need for a mating, female thread.
After boring the needed ID, I would single point cut the threads with an appropriate tool (proper angle and flat at the tip), checking the fit as the cut progresses.
If you guess at the ID, try to guess on the small size as you can always cut more off, but putting it back on is difficult. Not many lathes can do that trick. The problem with using an unknown ID is that you can cut the threads too deep if you cut a sharp peak down below the original level of that bore. Then the pitch diameter WILL be too large and you WILL have a very loose fit.
That flat on the ID MUST be correct when you make any trial fits to see if the thread is cut deep enough. If it is not correct, then a sharp Vee at that point will interfere with the fit and cause you to cut the female thread too deep. Just ask me how I know.