Not the axis. Just the annotations. You can actually zoom in and get any level of precision you want on the x-axis.
I think above the "Graph area dimensions" would be good. I would not us the RT abbreviation here, even though it is tempting to reduce the length of the label, but the axis says "Retention Time".
I guess I would use "Retention time annotation digits:" It is a long one, I know. If you use "precision" then you would need to add the word "digits" after the edit box to clarify what the number actually is, because "precision" is 1 or 2 or 3 doesn't make sense without the "digits" to clarify what the number really is.