Hey Sarah,
I apologize for the delay in my response. I was doing some more investigation on the topic.
Unfortunately, I could not find anything in the platform's code that indicates that the calculation field had the capability to display that warning message.
On the other item, this error message is very specific and is only happening in a special use case. Otherwise, you should be able to freely adjust the formula without the need to remove the field from all other custom forms.
The special use case is only there when you have the same custom calculated field with the same calculation formula on 2 different custom forms attached to a single object. That is there to prevent having the same field with different formulas on a single object.
I hope this is helpful, and I'm happy to jump on a call to walk you through these details if you're interested.
Thanks,
Gevorg