I've got a product that gives authors easy lookup tables for mass ratios...$3! Cheaper than a cup of coffee! Or a bottle of dihydrogen monoxide at an airport or concert venue!

Oh, I can do mass ratios. It's just that in this case, it's an unwanted complication.

Heinlein had a odd quirk, which is that he could do mass ratios as long the power source was moderately shitty (chemical, single-H, NTR) but he seemed to think they didn't apply to total conversion drives.

This is actually an area where slide rules would probably be pretty useful.

My slide rule is upstairs. On average, my computing devices are where I need them.