i should be venturing into the world of self leveler...need input

Hey folks, remodeling a bathroom and i have already demo'd everything last weekend. The last day was just scraping all the extra thinset off and checking the flatness (not level) of the floor. I noticed a few dips and humps as the house is 9 years old.

i will be doing 12x24" tiles and will be using schluter ditra as the substrate. i have tiled before and never considered flatness and didnt have an issue. this time i want to do it right from the start. should i use self leveler? i am thinking yes, but have questions regarding how to prevent it leaking down joints. can i just caulk all seems and use 1/8" plywood to make edge barriers?

also how do you find out how much leveler i need? i am trying to not increase the height too much.