The simplest way I know is to make a full-scale cross-sectional drawing of your mirror and telescope tube seen edge-on. Then draw a triangle whose base is the mirror diameter, and whose apex is at a distance from the mirror equal to its focal length. Next, draw-in the outer edge of the telescope tube. You then have to experiment by drawing a 45-degree line, representing the secondary mirror, at various points along the axis of the telescope and reflect the lines from the primary mirror's 'triangle' so that they are outside the telescope tube. Enough clearance has to be allowed for the eyepiece focusser. Place the reflected apex of the primary mirror so that it about halfway between the minimum and maximum throw of the focusser. Once you have the focus where you want it, measure the length of the 45-degree line and this gives you the major axis of the secondary mirror you need to use.