@marko and @audreyh1:
audreyh1 has it exactly correct. In my case, it appears that, since I have 21 years, I can do conversions anywhere between about $9K and $50K, and that can move me two or more brackets in my 70's. The savings is, well, take a look at the width of the middle tax brackets and multiply that by the differences between two of the middle brackets. It's serious coin that can go to my kids rather than the IRS.
Another thing is, for me anyway, I'd like my income to be low for the next few years while my kids are in college, so I am limiting my conversions for some of those 21 years. I was curious to know if I was getting myself into a high-tax situation from which I couldn't extricate myself.
But I see marko's point as well. I start looking at RMD's at 70, IRMAA, SS, kids' college, lifetime consumption smoothing, the probability of me even being alive to spend the money, the 4% rule, the market, asset allocations, the probability of Congress changing the tax laws again, and so sometimes I feel like I just want to take some very simple actions (like convert to the top of the X% bracket) and then go do something else because chances are I can't perfectly optimize everything all the time.
@Luck_Club, my grandfather, a tax accountant, had the same saying.