DEPENDS

IF the raise is one of the main reasons of job switch, the 20% is a good number. e.g. you earn 8w, a 8.8w job is NOT good for you. After tax, no big difference;
ELSE IF you are not happy with your current working environment or you want to learn sth. with your new position, the 10-15% is accpetable;
ELSE IF it's for other reason, any rate is ok. e.g. if you get a job neared to home, travel time is reduced from 1hr -> 15 min / one way, then -5% is even is ok as you'll get 45min x 2 = 1.5 hr more time, spending with your kids/family.