Hii, I have measured the distance from an estimated location of the air gap between the objective elements, to the eyelens of a 25mm unitron eyepiece at the "in focus" position as being 154cm. This is viewing straight thru, ie no diagonal. The Unitrons use a draw tube to get rough focus and then with a very short, approx 25mm rack, you get final focus. I guess u will need to deduct the length of the diagonal light path and make allowance for the focuser assembly u will be using. That 154cm distance is the critical figure for you. If you need any other information just ask
Mark