[Chak]

The calculator you used appears to be correct, assuming #25 chain and .25" pitch. With same size sprockets, ((2*C-C)/pitch)+sprocket teeth = #of links. This is just top length of chain + bottom length of chain + chain wrapped on first sprocket + chain wrapped on other sprocket. The top and bottom lengths = the center-to-center distance, so divide that by pitch to get number of links. The number of links wrapped on 2 halves of equal sprockets is the number of teeth on the sprocket. For your calculation, ((2*12.25)/.25)+18=116. In theory the chain would've been fine. There was some other problem in your setup.

As a rule of thumb, simply make the C-C distance a multiple of chain pitch and you will end up with a nice, even number of links. This is assuming same size sprockets with an even number of teeth, which is what we always use on drivetrains. Every #25 sprocket sold by Vexpro has an even number of teeth anyways. Only when sprocket sizes are different would I turn to the calculator.

As Bazel A suggested, adjustable tensioning is the proper way to make sure the chain continues working even as it stretches. But since FRC robots don't have to run for all that long, some teams forgo adjustable tensioning with no problems. They just use exact C-C distance, or add .018"-.02" to the C-C distance to account for the chain stretch. Done properly, this setup could last your robot through a full season.