ikev20
Put your current weight in the middle of the adjusting range, then add washers until its balanced, then add the weight of the washers and the current counter weight's weight together and you have the weight needed for your cart. you'll have some flexibility in variances because you measured in the middle of the travel range so getting one close should be close enough.
My thinking is a turntable manufacturer cannot know about every cart on the market. so do the leg work your self and get what you need.
I just ordered a Prime 21 as well and have a very light cart (Early VDH) I'm hoping will work if not I'll be buying one that matches the compliance of the arm.