The ideal TA level depends on several factors. If you are using acidic chlorine sources, such as trichlor or dichlor, keep TA on the high side, perhaps between 100 and 120. If you have a SWG, or if you commonly run water features such as a spa, waterfall, or fountain, keep TA on the low side, between 70 and 90. Pools with plaster surfaces should factor their calcite saturation index into the preferred TA level decision. Pools with vinyl liners can tolerate high TA levels fairly well.
If anything rain would tend to lower TA. The test is often +-10 and sometimes less percise than that.