Check this website, it states the theoretical distance:
http://www.auf.asn.au/comms/vhfradio.html
Could have saved me and PigDog a bit of time if I searched for this first...
I suppose when you're not at the equator, and you factor in that trees, mountains and buildings exist, the seeing distance will increase.
Regards
PieterPan
(edited, typo)