In all honesty, it's arbitrary. This is why most contests who try to have a percentage-made-by-you, it's 50%, because it's easier to estimate half of your costume than it is...well, 80%. No one has measuring tools or even standards, it's all an educated guess.
Half I can usually do. 75% tends to be halfway between half and all...so 80 is more than that? IDK. It's a weird number and unless the director him/herself can say whether they have some kind of standard that helps them estimate 80%, you're pretty much on your own. All you can really do is go in hoping you pass, and if you don't, you're at the mercy of the director whose word is final.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com