Yes, it is true, with every casted vote the upvote value is diminished proportionally with your VP. There is a post which explains the entire process how the rewards are calculated in-depth, but I don't seem to find it right now (there are others but that one I found the best so far).
The 80% mark is usually given because during a 24 hours period the VP replenishes by 20%, so if you go below that mark you will need to take at least a day break to replenish your voting power. One big mistake newbies do is go with voting power very low, like below 25%. Their votes become worthless, and that's a very bad practice and doesn't help them at all.
You seem to have a good mark at 90%, that is if you are around more often than every 12 hours to vote (or if you use auto-voters or sell your votes when VP goes way up - I do that if VP is at least 99%, practically preventing it to go at full 100% at any time).
Thanks for being my follower! ;)