Ya know, it would be a courtesy to post a picture or at least a model # when asking for advice.
The link Gene refers to is pretty close to the SPI manual I'm familiar with.
Start on page 4, setting up the diamond holder, so the diamond is accurately referenced to center and to the given tool setting gage. (needs to be centered horizontally and vertically) After that, you can change or re-set a diamond to the factory gage and work with confidence to your angle and radius settings.
To set a radius, that manual is not quite clear.
The pin, #21, is a very tight sliding fit. SPI suggests tapping it with a plastic face hammer. Push the pin to the side desired, depending if the set up is for ID or OD radius. Use gage blocks or a mic'd adjustable parallel between the previously set (calibrated) stops one on each side, fig 4, to set the OD or ID radius desired. Visual perspective, facing the unit as drawn: You can see that if #21 is above the stop on the left side it gives ID, and if below the one on the right it gives OD. If #21 is centered with right stop contacting it above, and left stop contacting below, the diamond point, when set with the factory provided gage, should be dead on center vertically. (As per set up procedure for the unit, done as the first step before using).
smt