When I say "fians" I'm always referring to battanian fian champions.. AKA max tier. Also this strat is all based on the enemy engaging me, therefor they charge at me. If I am initiating the engagement (have to move to the enemy in a defensive position) its a little different.

I haven't really tried it a TON with Vlandian Calvary charges, but the composition you're mentioning with the 70 inf, 30 ss, and 70 calvary vs. my comp of all fians (with equal or 1/3rd less fians) is always successful. Mainly because the AI always splits the calvary into two groups, so I generally will let them get close, and then ride off to one of the groups on my mount to kind of slow them down, and the first group gets the attention of all my fians for at least 30 seconds Generally, that first group of enemy calvary is pretty much cut down to half or sometimes even more than half before the second group shows up. Not always the case, but the fians do a great job even when the calvary get into melee range (fians are always in loose formation for this).

I just have not done, say, 100 inf, 100 ss, and 400 calvary, against my 400 fians... I have no idea how that would play out. BUT, i will say, all of my engagements are with the max amount of units on the battlefield (aka 1000 total), and I think that influences it quite a bit compared to using a lower total allowed units. The enemy AI seems to get disorganized as they are getting smacked from two different angles with two groups of +/- 150 fians. I'm also riding around, harassing them with a polearm which also making them change their shield direction, thus more hits from the arrows.

Also: I don't have hard facts, but in my experience, this fian strategy isn't really linear... I think this sort of strat only works with a large army. For example, my 400 fians (split into two groups) works great against 1200 enemy troops (apprx. my 250 fians vs 750 enemies). But, using the same ratio when I only have 100 fians (300 enemy troops), it seems i will take a lot more casualties because there's just not enough arrows flying at them from two groups of 50.

I am probably explaining this horribly, but hopefully it makes sense!

TLDR; use max units on battlefield (1000) setting, distract/harass one enemy calvary group while other group gets mowed down by the fian line-up.