The general suggestion is to convert an amount in a particular year that brings your taxable income up to the top of your current tax bracket. However, be wary of side effects that an increase in your AGI can have that might bring your marginal tax rate above the tax-bracket rate. (Treat any increase in Medicare Part B and Part D from IRMAA as if it is an additional tax.) Also consider state taxes and any amounts that your state might exclude from income after you reach a certain age, often age 65. You'll generally get maximum benefit of a Roth conversion by paying the taxes with other funds. You certainly don't want to use qualified funds to pay taxes if you are under age 59½ since amounts distributed and not rolled over or converted will be subject to a 10% early-distribution penalty. Planning much beyond that requires extensive modeling and various assumptions about the future which may or may not turn out to be valid.
You can use tax software like TurboTax to do a projected tax return to get an idea of your marginal tax rate and whether it is influenced by side effects. The CD/download version of TurboTax, not the online version, is best for this.
