You could do what I did on my light touring bike and go with an 11 tooth cog so that I could drop the big ring to a 44 (and still have a top ratio equal to a 52/13). Pick the rest of the cogs and small chainwheel to suit. I set up with half-step plus alpine gearing so that 1 shift on the front equals 1 1/2 on the back so I have 16 actually different gear ratios with my 8 speed setup.