Okay, but 400 standing heavily armed mounted soldiers? Even 100 seems like a lot. How many men-at-arms did a lord keep around back then?
For the Komnenian Army circa 1200, we've got 30,000 standing troops and 15,000 mercenaries out of a total force of 60,000. Cavalry is estimated at 12,000 men, not including allied contingents from Antioch or the Balkans, so roughly 1 in 5 men were mounted but I can't find information on how many of those men would have been kataphractoi. The tagma units, which were all kataphractoi, seem to have numbered a few thousand each but there was only a handful of them.
So, taking the Byzantines as the gold standard for a centralized army of this time, we can expect an advanced, rich and centralized state to put about half of its realm manpower into retinues and to be able to afford to mount a little less than half of their full retinue. But standing armies of this type simply didn't exist in the rest of Europe at this time and it's hard to find figures on feudal manpower.
There are a number of standing armies founded in the 15th century, so just at the tail end of the game, in France, Hungary, and elsewhere. The Hungarian Black Army of the mid-1400's started out with only around 6-8k permanent soldiers but seems to have swollen to nearly 30,000 by the end of the period, most of them mounted (though probably as horse archers and gusars rather than as knights or kataphractoi). But it collapsed once the founder Matthias Corvinus died because the cost became prohibitive. The French established companies of the king's ordinance in the closing days of the Hundred Years' War, and while these seem to have been roughly a quarter heavy cavalry (and half dragoon) I can't find a solid number of how many there were.
So I guess the best I can say is that in the best case, heavy cavalry would make up between a quarter and a half of standing forces, which themselves could make up up to half of available manpower. But the biggest obstacle to establishing a large, effective standing army was a matter of income rather than technology or military doctrine. Those who could afford to do it, did. So maybe to balance things, retinues should have a small cost while not reinforcing, so that a large retinue isn't so easy to maintain in peacetime and would require high taxes?