Outcall rates tend to be higher than incall due to the added travel costs so why not just set up a price to include travel within a set distance and then state you will charge extra for any outcalls out of this area.
Also when doing this make the extra charges realistic so say add ?30 to your outcall price to cover normal travel and then an extra per mile out of that area
My outcall rate covers a 10 miles radius from my home address and I charge extra to go out of that area especially as my area is all motorway driving which I do not really like at night.
I personally would charge ?1 a mile out of the area you have specified as that actually works out at 50p a mile as you have to travel back as well.  Don't charge by time as some guys may be much closer but harder and longer to get to (unless you drive) so would find it rather cheeky being charged more for a shorter distance.

In my area I have a minimum booking for certain locations based on the time it does take me to get there - I get asked to visit a lot of out of the way places, that are not far in miles, but take an age to get there. I tried charging it by mile and other techniques but basing it on mileage didn't work, I was chasing around for hours in the car for one little booking, taking up an entire day when I could have been back at base seeing a couple of clients closer or incall.

I don't commit to a price until I've worked out if I can get there, and how long it will take (I work it out), then communicate this clearly to the client BEFORE accepting the booking. That way no-one is bound.