You are correct, that is what happens. I actually wnet over by a small amount this year, about $500 and am loathe to pay $150 on that minor estimating error, so I'll probably recharacterize the $500 to avoid that situation. Ok, call me cheap.
What I do is an estimated tax return in late December after capital gain distributions are paid and then determine my Roth conversions. I still have to analyze where I went wrong this year but I suspect it probably relates to the gross up of international dividends for foreign taxes paid or some other estimating errors.
In early December I started collecting data on Divs and CGs, to come up with a target convert amount. As more announcements occur, I keep tuning it. Unfortunately, some of mine don't actually post till Dec. 28, and I don't want to wait that late, as I need to fill out convert form online, print, and mail in, then watch account to see that conv. is done properly. I don't want to wait too close to end of year, in case something goes wrong, want time to fix it.
A growth fund posted a small Div in June, and they estimated a pretty good CG for end of Dec., but no Div info, so I estimated June's Div again for Dec. Oh boy, way off! They posted a BIG Div for year-end. That put us over $1500 over the top of 15%.
While looking at all this last year, DW said we can just add to our HSA early in the 2015 year to drop 2014 income down below the top, so that is what we are doing to back up.
Knowing that we had the contribute-to-HSA backup, I really ran the est. convert amount right to the limit.
I think of the HSA contribution idea as sort of a "financial reverse-titration"