ABC's,
You probably already have the wall thickness set on the plates which will govern the shear resistance, this wall thickness (and loads) will determine the moments in the tank then which you can use to size the the reinforcement.
I usually look at the "Plate Forces" spreadsheet because this spreadsheet gives the plate Moments in kips*ft per foot of plate width. If you look at the top of the spreadsheet in the window label you'll see the (per foot) designation. This then tells me how much steel I need at that location over a foot of width which makes it easy since I design slabs in 1-foot wide beam strips.
If it's a small slab and you don't need a lot of steel at the location of maximum moment then I just copy whatever steel is needed at max moment over the whole mat. To find the max moment sort the spreadsheet by Abs(max).
If it's a large mat and you want to get fancy with regions of reinforcement then I usually estimate/eyeball the regions using the contour plots.
Also, if you find you need a lot of reinforcement over a concentrated area at the max moment, then I usually might use 2-3 foot strip width in order to distribute the moment to the adjacent concrete to get the average moment per foot down. To get this average moment you might have to make a cut through the plates to get a linear plot of the moment.
Plates are tricky, I always go by the notion that stresses are smart, they will follow their way to wherever you put the resistance. So as long as you have enough steel in the slab to resist the sum of all the moment in the slab, then fine, if its underdesigned in a small local spot then that concrete will crack and weaken and the stress will find its way to other strong locations, this is the basis for ACI's moment distribution in slabs.
