For race car so lower ride height, less spring deflection under load due to heavier springs and the amount of droop are the main considerations. I have no idea how far the car sat down on the original springs/dampers at static ride height so without that the length of the new coilovers is guess work isn't it? Reducing the static ride height by say 1"/25mm would be a good start but how do I get there? To make things more interesting the new setup has less travel.
I'm thinking that to reduce ride height by 1"/25mm the overall strut level should be reduced by twice that to take account of less spring deflection, does that sound a reasonable guess? It looks like the sleeve should be just tack welded in place until the powertrain etc is installed and the car can sit on it's suspension.
At least our resident racer would have struck and solved this problem if only when working out the amount of droop required so I'm expecting some informed responses here