DIY Swing Set. Need angle measurements
Without knowing the design of the brackets this is impossible to answer, but rather than calculate angles using trigonometry, there's an easier way:

Assemble it, and sit it on the ground. Then use an offcut of 4x2 or whatever, laid on the ground next the posts to mark a line parallel to the ground on each one. Then take it apart, cut them, and re-assemble.