Is it standard to ask for an "equal contribution" footnote in a 60%-40% scenario? I'm working on research that should be sent to publishing in the next couple of weeks.
The research is done by me and a friend of mine; we are both under the same professor. This started as an attempt to improve on another paper which I'm the third writer on, but it quickly turned into a new idea which we will try to publish as a separate paper; because of this weird sequence of events, we didn't talk about who takes the lead.  I'm taking tasks on almost every field of the research (building taxonomy, writing code, running experiments, writing the paper itself, etc.).
I'm also considering the amount of ideas and implementations that wouldn't be done if not for me.
I would say that overall my contribution is around 40%.

In my field, it's common to have an "equal contribution" footnote if two or more authors contributed equally. What is the common standard to ask for an "equal contribution" footnote?
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.