Jamie,

They WERE sort of preset from the factory. If you still have one rear and one front unchanged, you can just match those. Otherwise, you could compare and make them an inch shorter than the stock strut. The general rule for threads is that you have your MAXIMUM strength when the thread engagement is 1.5 times thread diameter. So, in other words, if that thread is 20mm diameter, at 30mm of thread engaged you have MAXIMUM strength, and threading it in further will not gain any more strength. I would think even 1" of thread engagement is going to be enough.

Ziggy