Mike: Having been there before, and gotten it wrong, my preference is to build a scale drawing that accurately shows you what's going on in detail. I built a beatiful hexagonal plywood tube and then drilled the hole for the eyepiece holder in the wrong place. I was able to plug the hole and redrill it in the right place and the patch just barely showed and no one ever noticed it but me to whom it was a persistant reminder of the misadventure. You can get the exact focal length of your mirror from a Foucalt tester. They are easy to make if you have some bench space and some modest tool skills. Or you might ask Steve Dodds to do it for you. The tester will also show you the quality of your mirror's figure and you can even record a picture of the figure on a DSLR. The position of the focal plane within the barrel of an eyepiece can be determined from the manufacturer in some cases and by direct inspection in other cases. With Plossls, just turn it over and look inside for the position of the field stop. For eyepieces with a field lense in front of the field stop you need to get specs from the manufacturer. Televue has this on their webpage. In general on modern eyepieces the field stop is within a half inch of where the barrel meets the top extension of the eyepiece. Lastly the the scale drawing will you how well the secondary is doing its' job. A common mistake is to buy a secondary that is so small that it blocks light from the edge of primay from reaching the focal plane. This is often done is the mistaken belief that it will improve the image by minimizing the obstruction of the primary. What it actually does is turns your 13 inch telescope into a 10 inch telescope. The unscrupulous will use this to mask a turned down edge on a poorly figured mirror. DT