Um, Murray, if the rear of your monstrosity won't pass through your Speed's front standard, it almost certainly can't be focused to infinity at 50 mm. Yeah, the rules are the rules. There's little magic about zooms.

More seriously, buy yourself a good book. A. A. Blaker's Field Photography, to be exact. Out of print, look for a used copy with www.abebooks.com, www.addall.com, and www.amazon.com. It contains the magic formulas you need and even explains clearly how to use them.