I think this is starting to get through my thick skull thanks to a very helpful
thread over at Rennlist and
this post in particular from Brian Broderick that explains the trig that goes into the effective rate calculations.
Here's where I'm at:
Front motion rate on our cars average around .94. Square that to get the multiplier for the front effective spring rate of .88, meaning a 250# spring up front will equate to around 220# wheel (effective) rate (250*.88=220).
The rear motion rate averages around .65 which squared comes to .42 for the rear effective spring rate multiplier. If I want the car to be balanced in the rear at a 220# front wheel rate, I need to work backwards from there.
If I keep my 23.5mm torsion bar, I can subtract it's 126# effective rate from 220, to leave me with 94# to make up for with a coilover spring. Because the multipler for that is .42, that means I need a 225# rear coilover spring to make up the 94# difference at the wheel(94/.42=223.8).
If I want to delete my 23.5mm torsion bar, I need a 525# rear coilover spring to maintain balance (220/.42=523.8).
As my order with Paragon currently stands (250/250) if I keep the rear torsion bar I would be looking at 220F/231R at a ratio of 1.05 back-to-front. That's pretty balanced, but would be almost exactly 1 if I drop the rear spring rate to 225# (not too late to change my Paragon order).
My other option is to change my Paragon order to a 525# rear spring. I will then of course want to double-check with Paragon on the valving for that rear 30-series Koni.
Because the latter doesn't cost me anything more, I think that's what I will be doing. I like the idea of never having to reindex the torsion bar and it'll make it easier if I ever turn the car into a track car and want to be able to easily swap spring rates.