Something I should add, that I forgot to mention, is that when I calculate percentages, I use the Canova method. I made an a excel calculator that does it for me, but for example, for 80% of a 4:00 mile, instead of doing 240 / 0.80 to get 5:00, I would do: 240 + (240/100*(20)) to get 4:48.
So there are slight differences depending how you're calculating. I did it that way and built my calculator that way because it just makes sense to me.