You need two things to determine GPM, head loss and the pump head curve. The pump head curve you can find in your manual or here:

http://www.poolplaza.com/Max-E-Pro-techspecs.shtml

Head loss is usually between 45-65 ft of head which means your flow is between 110-135 GPM. Your probably near the lower end given the size of the pump (higher head). If you want a better estimate then you need a more accurate head measurement.

Total Head = Return Head + Suction Head
Return Head = 2.31 * Filter PSI
Suction Head = 1.13 * Pump Suction (in. mg.)

If you can't measure suction head which requires a vacuum guage, use this formula:

Total Head = 3.2 * Filter PSI (assumes suction is 40% of return head)

2 1/2 HP seems excessive for a circulation pump. Your turnover will probably be less than 2-3 hr.