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)
I found Mark's post. So, if you take his simplified calc and use 108 as total head (108 / 3.2 = 33.75) your filter pressure (clean) would be around 33-35. Is it that high?