After completing inversion for a week I can say that if I hold my hair one way it looks like I've gained an inch. If I hold it a different way I haven't gained anything.
To cut a long story short, I really can't tell if i've made progress but I'm assuming I haven't.
I think I need to improve my length check techniques, I read a great post here about length checks and will follow thing method in future.
Have you had success with inversion?