Or just loom the item you want to use (that's my suggestion)
Foot armor is the most important for cavalry, as has been mentioned already your feet are hanging down the side of the horse, they're the prime target for getting hit.
That's why my favorite armor is the heraldic mail with surcoat, has 18 leg armor by default, it's (IMO) the best horseman's armor.
I would also use mail chausses (27 base armor) , but I switched to mail boots (35 base armor) a few months ago and I seem to have noticed I take more damage. It could also be that I loomed my heraldic mail so it's now 23 base leg armor, so I've netted 10 more leg armor total in my new setup.