Personal factors, your tax bracket, expected future income and when you plan on retiring all play a role. The answer isn’t as simple as playing the tax bracket game (convert be low a certain tax bracket only).
Other considerations can affect you taxes down the road. Even Medicare premiums are an issue. A high required minimum distribution (RMD)(currently starts at age 72, but pending legislation will gradually raise that to 75, if passed) can cost more than just a tax bill. It can also increase your cost for Medicare.
The best way to handle a Roth conversion discussion is to break it into two parts: the conversion phase and the retirement phase.