i made my shackles to give me 2.25" of lift (which it did) and in order to calculate dimensions to achieve desired lift all you do is add 2X the amount you want to lift to the shackle. So for me i added 4.5" to the stock shackle lenght (which is 4" from centre of the holes) so my shackle was 8.5" from hole to hole. one problem i encountered when lifting with shackles tho is that the leaf springs become much more susceptible to flattening due to the longer shackle which allows the spring to stretch out further then the stock one. I added an add-a-leaf a few months after i lifted it when i realized my springs were flattening out significantly.

hope this helped

dave