so the reason we're mentioning the ball is a compare and contrast thing: you don't need the ball, you just need to know where your load is relative to where a *ball would be if you installed it.*
so, to reiterate:
1) calculate the load on a 'virtual ball' for the maximum trailer allowable for your receiver. for that you will need to measure/calculate the distance that the ball would project from some datum (say, the locking pin, or alternately, the rear set of attachment bolts)
2) calculate the torque that 'max load' virtual trailer places on the mounting points to the vehicle, using the distance of the ball from the datum as the lever arm. Note this as the permissible torque for your hitch/vehicle combination.
3) calculate or measure the mass of your new rack and its contents.
4) calculate or measure the distance from the SAME POINT on the hitch receiver (as your virtual ball) to the center of mass for your new rack and its contents.
5) calculate the torque generated by your rack at its lever arm, as seen by the datum.
6) apply a 'just in case' multipler to account for the vibration and shock of rough travel.
7) compare the permissible torque recorded in step 2 to the final number for the new rack generated in step 6.
8) if the number on line 6 is less than the number on line 2, this is your margin of safety, and you may proceed with due caution.
9) if the number on line 6 is greater than the number on line 2, you will need to redesign.
Welcome to the wonderful world of educated guesses and compromise that makes up the engineering discipline.