“cosine gain” as a term might make sense in a universe where in all planets that had an atmosphere, the energy available in the atmosphere as a function of density and wind speed only ever increases the higher you go. In our universe, that’s not the case. There is not always more energy available at 200 kilometers up than there is at 0.5 kilometers up.
What you do in our present universe is to measure the wind speeds at different altitudes and design your system to target a favorable and achievable altitude, keeping in mind our current understanding of “cosine loss”.
What you are trying to get at is better said by just saying: look at the wind measurements at different altitudes and try to target the altitude where the wind speed is favorable currently or has been over a period of time.
If you want to introduce a new term, do your homework and quantify it, qualify it, and support it with empirical and theoretical data.