Is it standard to ask for an "equal contribution" footnote in a 60%-40% scenario?
I don't think there's a common standard for how to settle this after the fact, at least not one spanning all fields where author order matters. But my rule of thumb would be this:
If your estimation of your coauthor's contribution exceeds your estimation of your own contribution by more than the (likely also estimated) margin of error of your estimates, then you probably shouldn't ask for equal contribution. If the balance is too close to call, or the contributions are too difficult to compare (which is often the case in a theorist/experimentalist collaboration, for example) then sure, having a discussion about it is appropriate. (Just be open to updating your estimates based on the discussion.)

You could also consider including an "author contributions" statement in the paper to clarify who did what.