10 Units +5 per 100 points it usually a reasonable number; 15 per 100 is not.
Reinforcements can be whatever you want, just consider what all someone could get with those points and if you think it’s reasonable for the scale of battle you’re doing. How much of an impact do you want those units to have? (If you wanted the battle to be a little more historical the starting force would actually be the smaller one and made of recon units, with the main force coming later.)
So if you’re using 300 points / 30 points of reinforcement, I can get a PzIV G, or 6 SS-PGs, or a Sherman +9 points of soldiers, etc etc. It won’t likely be a huge difference in the battle but could be welcome aid. Reinforcments equal to 10% of your starting force doesn’t seem unreasonable to me.