I've seen a web page with leaf spring calculations on it, but unless you know the exact steel properties and what not, its better off just to get some junk spring packs and start playing. Pull them apart, turn them upside down and stand on the leaves, measure the deflection. Compile a table of the spring rates and lengths and then put together a pack and give it a try. If you need more or less lift, add or subtract leaves or swap siffer for softer leaves, etc. Repeat until you get the height and ride you are looking for. That's how I did a few packs I have run on my truck.