Hi. 2 things about Lightmix:
1) - As you already found out, rising the level will result in more noise. So in general you shouldn't rise the level, but lower it. So set your light a bit to high or at their max level, and then lower the values in lightmix.
2) - You can send your lightmix changes back to the scene by the push of a button, so you don't need to know the lumen values to do this (can't remember the exact name of this function, and can't open Max right now).
Hope it helps