For the arbitrary changes, I mean that I will sometimes add a clause along the lines of: "Includes a maximum of _ additional hours for work not already defined

For the arbitrary changes, I mean that I will sometimes add a clause along the lines of: "Includes a maximum of _ additional hours for work not already defined as part of the Deliverables." Where the number of hours is a very small percentage of the overall project estimate. I find it's an easy way to give the customer more budget certainty upfront and offer some level of flexibility. So as the customer asks for something out-of-scope that's small, I can just draw down the hours pool instead of debating between issuing change orders and eating the cost. And then you have some form of contractual protection saying "oh hey, your 10 tweaks took up the entire pre-agreed contingency, it's time for a change order" which is a much easier conversation to have.