If you can't tell, yes atmospheric drag is in effect (below about 80k anyway, and only for the active ship actually, above that value they've actually had issues with Physx not liking such tiny tiny tiny vectors.)
Why reinvent the wheel
Your other option is to just press M and look at your current orbit
, or is that not in the free version? It will even show you exactly when and where you'll transition to a different sphere of influence.