I took a little different approach this year, as I went through the same drill. My income after the standard deduction and exemptions are figured to cross over the 15% tax bracket. In rough numbers in my case(MF Jointly), if the threshhold for the 15% bracket is 63,700 and standard deduction(in my case, home is owned free and clear) is 10,300 and exemptions amount to 19,800 (4 kids), then 63,700+10,300+19,800=93,800
If I make 95,000, then I take 95,000-93,800=1,200 and put that in the 401k. This, in effect, drops me into the 15% tax bracket. Any amounts beyond that I put into the Roth 401k.
YMMV, but hope this helps you look at it in maybe a more objective way.