car - ae86
use - street+drift+touge+track+collect dust and rust
problem - how to figure what shock length

so car is lowered with some 200mm -220mm springs in the rear of the ae86, forgot exact height of springs...

anyways, yesterday while putting my S series back in teh rear, putting the shocks back in place made me relise how to figure what the length of shocks im going to need but would like someone to check with my thoughts

so when car is on the ground, measure the shock height as its compressed...for example....compressed is 40cm...so ideal shock length at full extention should be about 50cm-60cm?....10cm more...

by looking at the spring being compressed..i think the springs only compress atleast 1-2cm max as its rated 6kg springs..

is this an ideal way to figure out best shock height to suit spring height?...i know theres a proper way but ive read threads and shit and it doesnt make any sense to me so im trying to make it as basic as possible