Bicycle Mechanics - Chain just a tad too tight

Single speed setup on a road bike with vertical drops.
Chain is just a tad too tight.
Current chain ring upfront is a 48.
I have noooo idea how much difference taking away just one tooth will make if I change it out.
Should I try a 47 or 46 or 45 or.... ?
I know you can't tell for sure without seeing it but maybe you can give me some kind of idea?

And which would make a bigger difference? One tooth less on the front chain ring or one tooth less on the rear sprocket? Current rear sprocket size is 20t.

The rear is an i-motion 9 speed internal geared hub so I can play around with sizing more than a standard single speed.

Thanks!

The difference in 1 tooth will be about 1/8" in the chain (1/2" total but split between the top and bottom sections of the chain). The difference will be the same if you change the cog or the chainring. The best solution is probably a half link (http://harriscyclery.net/itemdetails.cfm?ID=1274).

Thanks!
I actually have a half link and if I install that the chain is too long.
I was hoping to get somewhere in between.

Consider this, each link in a chain will mesh with a single tooth on the cog or chainring. Adding a half link is the same as subtracting a single tooth from either the cog or chainring. It sounds like you will either need to run a chain tensioner like the surly singleator (http://surlybikes.com/parts.html) or try a different chainring/cog combo that will give you a similar ratio. Perhaps 46/19. But there is not guarantee that it will be any better. You might want to give this a shot: http://eehouse.org/fixin/fixmeup.php

Ok, cool.
So after I input my info, any gear combo in the highlighted column will give me the correct chain tension?
If so, it looks like it's telling me that I just have to get the bring the front ring from 48t down to 46t and take 3 links off the chain.