I agree with Adriano. I started off with a 16T rear cog and had a choice between 52T and 40T chainring (as these were the two available on the 10-speed I was converting).

I tried the 52-16 first, 88GI, and while I could get around and conquer most hills, I was tiring out really quickly and had quite a hard time stopping.
I swapped the 52T for the 40T and have been cruising along a lot more easily on the 67.5GI. These are kinda opposite ends of the spectrum, and I spin out on even mild (sustained) downhills with the 67.5, but I find it's much more manageable overall, and I keep a much better tempo when cruising on flats and rolling hills.

I think a low-to-mid-70s would probably be perfect, and that's what I'm aiming for when I get a new cog - maybe a 40-14, or something similar.