Phil don't forget you fins under you boat 
 
 we usually add  "Inboard Bunks" to lifts we install 
this will keep the fins a safe distance from the cross rails 
additionally we'll add either a bow stop or side rails or bumpers 
so you do go in too far......... on my lift I have a piece of 1" pvc attached 
vertically and lines up with my throttle lever, then I know the boat is in the right spot
water depth min max will also vary with the type of lift you are getting 
a vertical lift has a much longer / larger lift height and some models can go all the way down to the 
lake bottom, cantilever type lifts have a much much shorter lift height and require more min depth than a vertical
bottom line it's basic math to figure out a good starting point , then a little trial and error to get it just right 
good luck 
Dan T