20-25mm drop is acceptable as that's what I currently run with the stock R&T spring rates. Im around 14.5 inches hub to fenderA 9k 8 inch front spring needs 63mm of compression to support the weight of the car. This would give a 20-25mm drop from stock performance.
Moving to a longer 9 inch spring would give you -10 to +15ish range.
I assume if I go this route, I reset the front to the default 245 mm shock length and adjust preload to get the correct ride height?