I apologize in advance if this is a moronic question because I already know it's not a complicated calculation. Unfortunately, the last time I did this was at least 10 years ago, and I can't figure it out anymore.

We want to switch from making post-tax deductions to Roth IRAs to pre-tax deduction to a 457b. Right now, we contribute $800 a month after taxes. I want to contribute as much as it will take to a 457b to "feel" like it's $800 missing from the paycheck, but not more. Because those will be pretax deductions, they will lower our overall tax taken from each paycheck, but how do I figure out how much that amount is? We're in the 25% tax bracket. I completely understand the concept of marginal tax brackets, but I think that's irrelevant in this case.

What's the formula for figuring that out?

Thanks!!