yah, the lookup table is just discrete points of a stepwise function, so why not just use the underlying formula instead, and smooth it out while we're at it since it simplifies the calculation?

in this era of handheld supercomputers, we're well beyond having to manually multiply out the formulas anyway. and tax forms are already basically one long formula where you just plug in the variable values in a step-by-step manner (and usually have a computer do the math for you).