different glass works differently, different weather conditions. make sure you rinse better. I regularly go up to 15, and on certain types of glass, in direct sunlight, very rarely will start to get light spotting.
but recently I mistakenly went up to 27, and in dull drying day windows were perfect. even when sun came out and doing insides. I wouldn't like to risk it tho regularly just in case. but perfectly happy to go up to 15.
not so good in direct summers day heat, on non sheeting glass. but so long as plenty of rinsing.....