for those who don't want to go down an orbital mechanics rabbithole, the mean distance over time is a(1+(e^2)/2)
where a is the semi-major axis and e is the eccentricity. So it's 0.15% farther than the semi-major axis on average

But since this resolves based on instantaneous distance It's probably better to plug the perigee into the black scholes formula.

What distance? Perigree, apogee, semi-major, semi-minor?

@MRN267 Instantaneous