I am building a hardwood fence with 100mm posts. A contractor set 90mm post stirrups (don't ask) in concrete on the fence line as part of a spoon drain. The stirrups were level when placed, but have set off level. If we placed the posts in the stirrups as is then the fence would lean forward. What's the best way to cut the 100mm posts down to fit the 90mm stirrups so that the fence will be straight?