Start from the behaviour, not the percentage
Write down the three behaviours you want more of. Common answers: more new accounts rather than repeat orders from the same familiar customers, collections within terms, and less discounting. Then check what your current plan actually rewards. In most businesses the answer is revenue at the moment of booking, which encourages precisely none of those three things and actively discourages the last two.
That gap between what you want and what you pay for is where most commission plans go wrong. The percentages are usually a secondary issue and they get all the attention because they are the part that is easy to argue about.