The problem here is calculating ΔV as the rocket travels through the atmosphere, since the change of atmospheric pressure affects the rate of mass flow (ṁ), which in turn affects the specific impulse of an engine. There's also the difficulty of accounting for the constant change of the wet/dry mass ratio as you burn fuel at different stages of your ascent: depending on your launch profile, you might need more or less fuel with the same rocket to get to orbit.

It also depends on how efficient an engine is for atmospheric conditions: engines whose specific impulse don't change that much from sea level to vacuum are able to attain a larger change of velocity while in the atmosphere than engines that change more drastically.

Calculating the remaining available ΔV is much easier once you're in vacuum conditions, and in most cases, it's only the core stage that deals with a non-negligible atmosphere, so you can ignore the sea level calculations for upper stages.