'Best' Gear inch to start with on a fixed gear? Moderatley hilly area
I'd start with a 70 gear inch ratio for a while and adjust from there. Then change your rear cog by one tooth at a time till you figure out what works for you. I would stay within 65 to 75 gear inches for most on-road riding. You could get away with 75+ gear inches if you lived In a flat area or were just a beast of a masher.

The problem with fixed gears in hilly areas is that you'll have to spin very quickly when going downhill.  Keep that chain tight and your momentum will help on the climbs. 

I prefer to ride slightly higher (about 74 gear inches) since I live in a hilly area and want to be safer when spinning on decents.  It may take a few weeks to get used to riding fixed, but your legs will adjust quickly, so don't be too hasty to switch things up.

Also, consider the skid patches with your setup. You don't want to have a setup with only 2 skid patches and wear down your tires too quickly.  http://www.Bikecalc.com has a good gear inch and skid patch calculator to help figure it out.