There are thousands of knights when the Ironmen are put down under Robert. If there are about 400,000 troops in westeros what’s a good knight to common soldier ratio? One percent knights seems too low but 25% seems really high, if commoners outnumber nobles 100-to1.

A couple things:

1. 100 to 1 is off. In Medieval Europe, the nobility was almost never only 1% of the population. 5% was more common, although in some places it got as high as 10%. 
2. 400k is the fighting men of Westeros, where the nobility are disproportionate. Pretty much all able-bodied noblemen in Westeros learn to fight, whereas only some smallfolk ever do. 
3. 25% is around what we see in the armies – Robb’s heavy horse is around 27% of his army, the Lannister horse is around 30% of their total number, Renly’s 20,000 cavalry is around 20% of his massive army (although I have issues with the numbers there.) 
4. However, if we take 25%, that’s 100,000 knights in the whole of Westeros, which has a population of around 40 million. In other words, they make up 25% of one percent of the population of Westeros, or roughly 10% of the male nobility of Westeros (assuming 5% of the population as highborn, and thus 2.5% being highborn men).   

So, no, I wouldn’t call it too high. If I recall correctly from the last time I looked it up, around 18% of the male population is in the military age demographic, so it’s a bit below what it might be, but you have to account for lords keeping some of their strength at home and trying to preserve heirs so that the House doesn’t take too much damage in the war, and that some percent of male nobles wind up in the Faith or the Citadel.