30 hours per week or 120 hours per month x $65 per hour/session = $7800 gross per month x 25% = $1950 in fees, leaving $5850 in your pocket (not bad).
Based on your $1500-$2100 fees, you would be paying $1.15-$1.61 per square foot for using the 1300 square foot studio.
As you know owning your own studio comes with a price. Not knowing what the commercial price per square foot is in your area, I would renegotiate your fees or offer to pay a flat rate/rent. BTW…who proposed the 25% rate?