Your train of thought is similar to mine. I haven't yet faced problems of this sort but I have spend time thinking and studying this. Anyway, there are three ways of playing and fixing this, that I know of. I'm ruling softer springs out as a fourth option.
In order of cheapest and easiest:
1. Spacers. If you have short stroke with spacer. Get O-spacer and regular bottom spacer. Example if your spring sets 20mm off the upper spring seat (20mm from captive). Get 20mm bottom spacer in the bottom of strut and 20mm O-spacer top of the strut. This will move the cartridge 20mm lower. You wont get 70/30 ratio anymore but 20mm in the most cases isn't nothing. Make sure that there is enough rebound left.
2. Helper spring. Easy and best but illegal(in Finland). Progressive springs kinda belongs in this category.
3: Height adjustable strut-tops. These have same affect as the spacer-trick. Basically these reduces rebound but these have more adjustment to play. Think about how easy it would be to make things match.
As you can see option 2 is the best route if you have serious dilemma with un-captive springs. 70/30 ratio isn't always valid depending on shock and its original application. For example Konis shouldn't be too overparticular about 70/30 ratio thing.
Do you think that you will have captive (captivetaty/captivitation OR woteve) problems with 5.1kg/mm(?) springs and short stroke setup. I mean, I have 4.0kg/mm springs and short stroke setup with 40mm bottom spacer and my springs are 30-40mm captive.
Just remember that when you haven't enough compression left - you might break your shocks but when you haven't enough rebound left you might break your whole car.
I hope this gives you some thoughts and feel free to correct me.
And once again sorry my bad English.