Salute!
A good question up front, and as one said above, altitude may play a role. For most of us, I don't think it would be too much of a correction , but you could crank altitude into the equation.
Secondly, the basic idea would be fairly close if you recall that the "mile" per "minute" of longitude will be the cosine of the lattitude. So where I live, it's about 0.87 N,M, per minute.
When planning some timing and geographical aspects of a few attacks back in the day, we assumed a "flat earth" within an arbitrary distance of the tgt, and corrected the longitude "x" values on our plots using the cosine of lattitude. Seem to recall we could get relationship of tgt and IP within 20 or 30 feet of real world compared to a very accurate inertial frame that used a geoid model of the Earth, altitude, phase of the moon, Milankovitch orbital theory, etc.
Thank you megan and Terry. Many lessons to be learned and taken to heart from that sad accident. I had no idea what really happened until I found the posts here on pPrune.
Gums sends...