Put the round stock between centers.
Alternatively if you use the compound method,... You still need two dial indicators (unless you have a DRO). The one indicator will measure your z-axis travel (position it on the bed with the point on the carriage), and the 2nd indicator also on the bed to measure the x-axis change of the compound with the point on the side of the compound. No stock is required. Change the angle on your compound until a movement of 1.000" in the z-axis, shows a 0.021" movement on your x-axis indicator.
Tip - I always like to move the saddle in the direction that increases x. Even if your compound side is smooth as glass, that results in a smoother reading on the x-axis indicator. But both do work.
You can cheat and just use the angle marks on the compound, but I usually find that they are not accurate enough for many jobs.