Make rules for wiggle room for compute unit limit?

See https://x.com/stegaBOB/status/1769732575508242742

> Q. you don't add any buffer?
> A. I think right now we have it as 1.2x the usage, clamped between 25k and 100k extra. For the sake of simplicity I didn't in that example. Left a comment that one should probably be added. Maybe I should be more explicit though 🤷‍♂️

We may want to include this in code.