The whole tub. Caution though, it does get slippery on porcelain. I've used it to scrub down my sink to get rid of the buildup.
I live in an area where we get well water 3/4 of the year and rain runoff the rest of the time. Mineral content goes through the roof! My neighbors put in a filter that uses table salt in place of the minerals and I thought I was drinking a salt lick.
It does a good job on mildew, but being a strong acid, use caution and definitely rinse everything down afterwards.
Good luck