Let me try to analyze the existing gearing a little.
Your front pair is 50/45, setting the granny aside for the time being. The ratio between them is 1.111. The incremental ratio for the 6-speed should then be 1.222, or a 22% jump between adjacent rear sprockets.
For a 6-speed cluster you have 5 jumps. Assuming 22.2% that's a total ratio of 2.727 (raise 1.222 to the 5th power) across a 6-speed cluster. The small-big pairings (ignoring feasibility for the moment) would be 11-30, 12-33, 13-35, and 14-38. I think these are all problem gears to get, unless Pastor Bob can make you something special. But you don't really need a super wide range double because you will have a granny gear. So let's think of a more standard road set up then add in the granny thinking.
A more reasonable roadie setup back in the day was 14-26. let's calculate backwards to see what the chainrings need to be: overall rear ratio is 26/14 = 1.857. The cluster incremental is factor is 1.132, much smaller than the 1.222. We'll look at actual tooth counts in a moment. For a full step of 13.2%, we will have a half-step of 6.6%. The ratio between the big ring and the middle ring must then be 1.066, so for a 50 tooth big ring we want a middle ring of 50/1.066 = 47 teeth. So your chainset is 50/47. Two teeth make a big difference.
Now we use the 1.132 factor to get the cluster counts in raw form: 14, 15.85, 17.94, 20.30, 22.98, 26.01. Rounding those, we have tooth counts of 14, 16, 18, 20, 23, 26. This at least does not have any oddballs. The gear range (27"wheel diameter) will be 96 inches down to 49 inches. if you add a granny gear of 34 teeth, you can get a bottom gear about 35 inches.
The chainrings should not be too hard to find used. You would need a front derailleur that can raise the chain from 34 to 49, a 15 tooth jump. The total chain wrap is 50-34+26-14=76-48, so your rear mech needs to wrap 28 teeth. But if the granny shift is good you could make this and it would give you 10 evenly spaced gears to handle all the normal road riding tasks.
Other optimizations are possible. For example, the granny can get bigger if the rear big sprocket gets bigger. If you can get a log-spaced 14-30 made by Pastor Bob, you can make teh granny a lot bigger and easier to shift. A good target might be to assume a 10 tooth jump (easy front-end up-shifting!) down from the middle ring, for a 37 tooth granny. To get the 35 gear inches you need a 14-28 in the rear. Probably the rear cluster would then be something like 14, 16, 18, 21, 24, 28. Now the main gear range is 96" down to 45" roughly. This new configuration needs a rigorous numerical check. But still, 50/47/37 and 14-16-18-21-24-28 would be a pretty decent half-step plus granny. Chain wrap is 27 teeth. I'm not sure if the 47 middle ring should still be a 47, but that is one reason to repeat the rigorous numerical evaluation.
The other half steps suggested by JohnE would also be pretty good.
Last edited by Road Fan; 02-04-20 at 12:37 PM.