Hi Jari

Good question. I tend to go by information in the botany books if I cant get my hands on a specimen. (always easiest to work from life). And, as time has gone by, by trusting my eye. In terms of scale, I try not to worry too much as my illustrations are shrunk and enlarged all over the place. And illustrating some things like trees etc 1:1 isn’t feasible. I tend to stick to an A4 size. If I were doing a new species description etc Id definitely be using callipers to be sure everything is exact, but mostly my work tends to be long lists of plants that just need to look and “feel” right, which is a slightly different aim. As long as I have the different parts of the plant in the correct ratio then it’s more or less ok.
Thanks for a good question though, mostly in botanical illustration scale is vital. And even for i.d. work like this, the proportions remain very central to identification and to getting the plant looking right.