One sure way is to take an EP and find out where the focus point is for the primary. Make up a quick mounting (maybe using the focuser that you are going to use) and just look down the tube and get a focus point. Don't be very interested in keeping out of the way of the light path. When you have the scope focused then you can measure everything and at that point you will know where to place the hole in the tube. Onaxial distance is everything in this case. Also allow for a bit of adjustment with the primary mount (that's the other way) by not drilling those holes until you have the front done. Good Luck!