There's where I started to go wrong; I assumed that the percentage would refer to the tribe as a whole, as the percentage for Storm Bull cultists above did.
Yeah, that's too few worshipers of the Orlanthi horse god among Orlanthi horse riders for my taste, so my Glorantha will vary on that. I'll increase the percentage of those with the Fire rune at the expense of the other elemental runes on the theory that those with it would be drawn to a horse-riding tribe more than those without it. Let's see, reduce the Storm rune to 70%, and the other four by half to 2.5% for a total of 20% with the Fire Rune, for a total of 500. Of those, make 40% Elmal worshipers, 30% Yelmalio worshipers and divide 30% among the other cults. That gives 200 Elmal worshipers, plus a few female worshipers. That should work for my purposes while still not altering them too far from the official version.