I think that depends upon how the stage rules are written. If it does not say you cannot change the position of the chair, then you can. If I had thought about that at a recent long range rifle match, I think i could have easily doubled my score on a stage. If nothing else, it pays to ask for clarification from the RO. Then again, assuming the rules do not specifically forbid it, "'tis better to ask for forgiveness than for permission..."