Re: Gear ratios for HF bike -- please help!
Sprocket choices depend on how much fabrication skill you have, and what soer of equipment you have to use.
You should do most of your reduction i the first stage. You are planning on using the bottom bracket bearings for a jackshaft, and they are not high speed items. I would really recommend replacing them with sealed ball bearings.
The 28T final sprocket limits you to a few choices, none of them easy bolt up stuff.
12/65 > 10/28 just makes it. A 65T #35 chain sprocket would have to be fitted to the jackshaft input, and a 10T sprocket on the output.
12/72 > 12/28 works too, but the 72T #35 sprocket is over 9" OD with a chain wrapped around it.