There really is no easy way to evaluate degraded tanks because local corrosion damage typically results in leak before break, provided the materials of construction have adequate fracture toughness. I presuming the tank was correctly designed to begin with.
Attempting to calculate a minimum wall thickness will not be realistic because in most cases with water storage tanks, corrosion damage only effects localized areas. Since this is an in-service water storage tank, you are probably concerned about internal corrosion damage from through-wall pitting (aka local thinned areas - LTA's) and eventual leaks.
I would suggest API 579 for evaluating LTA's in tanks. This is an excellent reference book for in-service component assessment but it is very expensive. Here is what I would do as another option, instead of attempting to over analyze this;
1. Remove the tank from service, and drain it.
2. Clean the inside surfaces and have a thickness survey performed by a nondestructive testing vendor using ultrasonic testing.
3. Based on the results of the UT thickness survey, you can decide on the extent of local repairs to the tank wall to avoid leaks. The repairs should be performed in locations where the LTA is below 50% of original wall for starters. If the damage is too extensive, you can decrease the repair scope threshold by using only 35% of orignal wall and monitor the remaining areas until you have some idea of a corrosion rate. At some point, the tank may have to be replaced or major repairs would be required depending on volume.
4. I would periodically inspect the tank to stay on top of degraded areas.
