Mechjeb Delta-V Calculations

[Zosma Procyon]

When MechJeb does Delta-V calculations in atmosphere, does it assume the rocket stays at the same altitude/pressure, or does it assume a more or less vertical launch?

[helaeon]

When you hit the atmo button in MJ or KER it's saying at sea-level only (so your worst ISP). So it's assuming that you never leave the ground. Your true DV to orbit will be somewhere between the VAC and ATM numbers depending on the aerodynamics of your rocket and your flight profile. Usually closer to the VAC number as atmospheric pressure drops off pretty fast.