I understand where everybody is coming from when they say that salary + 5% is very generous. Really, I do. However, I would caution OP to consider the following two (which it sounds like he has, at least for item 2 below).
First, biggest reason to go over 5-10% + market salary: if you think he will bring in that much additional money. Say you pay him $100k + 10%. Suppose your current profit is $50k / year (you said you were mildly profitable). If he ramps this profit from $50k to $200k, you're still ahead of the game. Yes, it is possible: with only 1 founder, OP is stuck doing a lot of overhead work, which would take less time of a second guy. Plus, there's:
2) The emotional support. Fresh blood can definitely help the weariness and solitude of running your own company. Sure, employee #2 may not bring in another $150k, but he could help you increase what you bring in due to more enthusiasm (and, more impt, quality of life).
If you ever want to take a vacation, you will need a second in command. Higher percentage means more (emotional) ownership, so this guy is also less likely to treat it as another job. He probably doesn't want to work for someone else ever again (a common thought), so he wants more ownership.