You screwed up the calculation. The formula 20*(number of stat increases)+20 is right; you just got the wrong number of stat increases. Extreme Evoboost boosts 5 stats by 2 stages each, so the total number of stat increases is 2*5, which is 10.

If you first add 5 stat boosts and then double them, then it can work if you do it right. But in your answer, you also doubled the +20 at the end, which is wrong.