If you already have eyepieces, try borrowing a scope that uses a moving-tube focuser of some sort (Schmidt Cassegrains and Maksutov-Cassegrains leave the eyepiece fixed and move the mirror--Newtonians and most refractors have a moving eyepiece tube like you'll have). By experimenting, find out which of your eyepieces requires the tube to be furthest in. Then measure how much farther out you have to go for your other eyepieces. You'll have to move your own focuser a similar amount, assuming that the 2"-to 1.25" adaptor adds the same height to your focuser and to the borrowed scope's focuser. The amount of racking in or out that you have to do between any two eyepieces is the same, no matter what scope they're on. So you can learn the difference in travel on another scope. Then leave a little extra travel on each end for future eyepieces or accessories.
Remember, you can always cheat a little and pull an eyepiece partly out of the focuser to extend the focus, but you can't cram it further in if your focuser won't rack in far enough. So err by putting your focal plane 1/4 inch too far out of the tube if you err at all.
--Dan Johnson