How to set max Roth conversion in constant dollars

Is there a way to use constant/today’s dollars for the maximum amount to convert? For example converting to the 22% bracket, but limiting the amount to an amount currently creates a decreasing set of conversions. My assumption is this is because it’s applying the cap in future dollars. I want an easy way to effectively tweak the brackets to try out conversions up to the 18% effective tax rate, for example.