Feathertouch focuser question.

I'm piecing together a 10" f/4.8 dob. I'm thinking about installing a Feathertouch focuser but I don't know which one to get. They offer 1.5", 2", and 2.5" travel versions. I'm interested in using a binoviewer as well so I don't know if that effects how much travel I need. Do you just pick the one with the most travel, or is it more complicated than that?

Also, does anybody know a good website that shows how to install and square a focuser? Thanks

The length of drawtube does matter. If it is too long and a good deal of it is in the scope (the light path)while you are viewing, you will reduce the amount of light you gather and might recieve some vignetting. I have a meade starfinder 16 in f4.5 and it needs a drawtube length of 4.5 inches, so I couldnt use a feather touch, I had to get a JMI focuser that could accomadate my needs.

To test drawtube length take the scope out without a focuser and measure the distance you have to hold the eyepiece out from the tube to get a focused image. (do this on mars and a deep sky object to get a good range). Then you can use those numbers to determine what a good draw tube length would be. When in doubt, go a little longer.