Does anyone know where I can find the formula to get a monthly retirement pay estimate?

I'm about to Medboard out due to multiple back surgeries. I have a total of 15years in. Most Guard time. I'm currently at a 60% VA Rating with a pending increase requested by the Dr. When my medboard completes I will be receiving medical retirement pay instead of a severance payout. Does anyone know where I can find the formula to get an average monthly retirement? Thanks in advance.