depends on the angle. which I know isn't helpful.
from an engineering perspective at 45 degrees the force's are about equal in the vertical and horizontal plane so you'd want to cut it half and half. as the angle increases towards vertical (lets call it 90 degrees) the forces shift to the vertical and vice verse as you move towards the horizontal (0 degrees). there is obviously a point at which any you don't want to reduce under with any angle (say between 1 and 89 degrees) which from memory is around the 2/3rds.
looking at what you've got I'd say your 2/3rds is acceptable.
isn't there a calculator for this on blocklayer?