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I'm not sure I've ever hear such a statistic on the correct amount of bar drop. I've also not seen evidence (or research for that matter) to suggest long term damage.

Anecdotal evidence: Of the women I've rode with, I haven't seen a significant amount of difference in bar drop.

+1. 10% is kind of a specific but dubious rule of thumb, considering that rider proportions vary, they have multiple frame sizes to choose from that could all do the job, and the amount of drop varies between different handlebars.

I've never calculated the percentage of saddle height that my bars drop, and never will. For a new rider, starting the bars level with the saddle and working downward with fitness and flexibility is probably the best "rule of thumb."

I'd let the woman rider choose for themselves .. just may not want drop bars at all ..

I have never heard of this rule of thumb. People are different and so bar heights are different. Same with saddle height rules, KOPS rule, 'top tube should be same length as forearm' rule, 'handlebar should obscure front hub' rule... etc