question about turning a groove

[kd0afk]

I'm having a brain fart, help me.
I need to cut a groove for a ball race. The center of the groove needs to have a 1.25" rad. and I don't have a center to go off of. Is this the right method;
Measure diameter of work, divide by half to find radius, subtract groove radius (1.25") and that will be my distance in from edge right?

 

[stevehuckss396]

I'm not exactly sure what your asking. Can you post a simple sketch?

 

[kd0afk]

Nevermind, I figured it out.