So I'm at a place 30 miles from where I flew this morning. Do I need to recall?
What kind of variations if any should I expect if I'm further away from where I calibrated last?
The acknowledged minimum is over 300 miles or, if environmental conditions exist that would require a nearly constant calibration.
Here in Florida, I've gone from a little outside of Panama City Beach to Tuscaloosa, AL, 260 miles, and had my P2V+ V3 hover there just as well as it does in my back yard. X, Y and Z axis all under 300 while Mod Axis right int he middle.
The soil differences are pretty significant. Here in my backyard, it's all non-mineralized sand. In Tuscaloosa, it's all red clay and deep Earth dirt with light to moderate ground mineralization.
The issue being, if your aircraft isn't giving you indications of being out of compass calibration, and your computer isn't giving out of bounds readings for the various axis, then you're wasting time doing the calibration. Some even say that doing so at each flight point is to risk calibrating over a magnetically confusing ground point. My aviation career would tend to agree with that assessment.
Do what you feel is right for you. Having said that, don't be surprised if you have a drone flyaway. Intense underground magnetic forces, such as from buried utilities, can interfere with a good calibration. If not connected to a computer to view the X, Y and Z compass readings as being below 300, you risk your own craft and money. The overall Mod axis is something to be considered as well.
I take a laptop with me almost every time I fly. I've had situations where people have called the cops on me thinking I've been "spying" on them. It's a simple thing to correct their way of "thinking" when they see the MicroSD card pulled right from the craft and inserted into a computer. That same computer allows for an instant, on-the-spot compass check that will tell you if your axis's are off. 268 miles North of my position with a significant change on ground composition showed me that nothing was needed for compass calibration.
But then, I'm a rather detail oriented person and would MUCH rather spend an extra 30 minutes in preparation than another DAY of flight time. My money is precious. So is my time.