how much result margin could be acceptable for small horizontal axis wind turbine fr
 

I want to understand how much margin could be acceptable b/w BEM method vs cfd. as BEM method include 2d effects while cfd includes 3d effect for wind turbine like tip losses and wakes.and if results according to bem calculation cp is 0.48 but in cfd cp is 0.35.i wanted to know what could be acceptable margin b/w two method.and if i want to improve the performance of turbine blades what are the possible solutions to look for or if there are losses how they can be reduced.