seems as though 275-300lbs is good for a 20XE engined astra.
There was a thread all about this a couple of months ago. Have a quick sufty through the history to see if you can find it. There was loads of info on there... or maybe that was about novas can't remember now... have a look though..
I've done this conversion on my hillclimb Astra. I'm using 6" 325lb springs but this gives a very hard ride and you might want to go for something a bit softer and longer for road rallying. You need 2.5" ID springs, not 2.25" if your'e looking at the TAS conversion kit. Demon Tweeks keep these in stock.