Zompist Bboard Again

I've been delving into the morass of Welsh numbers, with their seemingly limitless forms. One question that has come up is the gender of reciprocals. I've been looking for examples on the internet, and the results are mixed. For 'two fifths', I find dau bumed, so masculine, but for 'three fifths' I find not only the masculine tri phumed but also the feminine tair pumed. Am I looking at poorly learnt Welsh, or is it perhaps the influence of the feminine noun rhan 'part', perhaps fighting a dominating influence of the first three reciprocals (hanner, traean and chwarter) being masculine?

How ambiguous is "bob dau filfed eiliad"? I can see an argument by gender that it means "every two thousandths of a second", while "bob dwy filfed eiliad" means "every two thousandth of a second", but I suspect the gender selection may in practice be very variable. (The phrase comes from the description of a dynamic suspension system.)