Specific answers for your situation will require specific knowledge of your location, the species you have to work with, and the other environmental conditions such as soil, water, and aspect.  In general, you are seeking to concentrate the potential growth on fewer and better trees. Spacing by the numbers be helpful as a general guideline, but there must be some latitude for special cases.  Foresters measure trees at 4 1/2  feet above the ground on the up hill side, called Diameter at Breast Height (DBH).  General spacing is 5 feet apart for 2" DBH trees,  8' for 4", 11' for 6", 14' for 8", and the list goes on.  I prefer the crop tree release approach.  With this approach, you select your crop tree, then look around it to remove trees of lesser quality to free your crop tree's crown on 2-3 sides.  I always counsel people to avoid trying to fix it all at once and make it perfect.  Don't be afraid to put a lot of trees down, it will grow back faster than you think.  But don't look at it as a one time fix either.  Look at it as a process over time.