So if a student gets 450/500 points or a 90% on the lab portion, that's 45% towards their final grade. If they get 225/300 or a 75% on the lecture portion, that is 37.5% towards the final grade. So their final grade is 45+37.5 = 82.5% overall.

*edit*
Didn't see your edit, sorry! I'll check that out.