basically looks ok, collet size and tool length will vary quite a bit tho. I think thats why mills have extra travel on the column so you can set the optimum work height and tool spacing. A fixed gantry will always be a compromise.

Im looking at keeping the z travel reasonable(75 to 100mm) to reduce the cantilever and then either a deep bed and pack the work up to it, or be able to move the gantry beam up and down depending on the work piece. it means a little extra work but I will save on the motor, rails and ball screw. for the z axis. thats my logic any way

I think your diagram would look good in an excel sheet and then use pick lists for collet and tool size which would auto enter in the dims space, then add up the max travel :o)