Calculating challenge rating based upon a linear incrementation of the party's CR is stupid. There's a big difference between a CR 1 party taking on a CR 5 and a CR 15 party taking on a CR 19; this is because of how higher level spells and class features grant more actions to perform that handle different situations. Once players have access to the haste spell is an initial jump in their ability to artificially increase their CR, especially if there are a lot of martial characters. I've seen many times where a CR 13 party takes on a CR 20+ encounter and comes out with only 0-1 casualties; however, I've also seen cases where a CR 1 party struggles against a CR 3 encounter. To give you an idea of an appropriate upper bound on the CR a party can handle you should be more conservative with increasing the CR at lower levels and be more liberal at higher levels, depending on the optimization done by the party.