If you want to use a custom box for this, I would recommend using a dual-input Versaplanetary stage to fit both 775pro motors, following by a 5:1 or 7:1 VP stage. At that point you only have to do a small 3:1 or 4:1 reduction to get close to what you want at the spool.
Fully custom gearboxes shouls be avoided IMO if you don't have CNC machining capability of some kind. You'll end up with a heavier, larger product. That being said, integrating a Versaplanetary with an extra output stage to a spool can be made very compact and with minimal machining tools.
Look into the JVN Mechanical Design calculator instead of making your own calculators. Link:
https://www.chiefdelphi.com/media/papers/3188
The "linear mechanism" tab should have all you need, including projected current draw and other useful statistics.