I think such scaling as the author suggests, can be done in the head. 57g is roughy 3/4 of 80g. It's trivial to take 3/4 of almost anything. 250g -> ~180..185g, 40 -> 30, 50 -> 37g. Unless you do bakery, where proportions are very important, there's no need for 3 digit precision.

And yes, in general a slide ruler is a great tool. I should try it again.