According to ChemGeeks spreadsheet my water is balanced with these numbers

[chem geek]

OK, now that your CH number is more "normal", at least for your fill water, it is true that you don't need to go as low in TA (the 50 I mentioned). 80 should be fine and yes, some people run with more but you then run the risk of fighting rising pH, especially if you have water features or other sources of aeration. So if your final CH in your pool is around 300 and your TA gets to 80-100 you are in very good shape indeed, but don't worry about this combination being perfect (i.e. CSI=0) since it does not have to be.

As for doing a full refill in a safe way that doesn't have your pool crack from underground water table pressure, some people have found that they can use a large plastic pool cover that extends way beyond the edges of the pool (a very large plastic sheet, actually, not a solar cover). Then with this on top of the pool, they simultaneously drain the water from under the cover while filling with water on top of the cover. This way, you can replace as much water as you want without ever having a lower water level and doing this efficiently to use the least amount of water possible. Really neat and I wish I could remember who said it or where the post was since I'd love to give credit where credit is due (I just love clever ideas). You could probably use a solar cover for this technique, though there will be some mixing at the edges (hopefully not too much).

By the way, how in the heck did you ever get your pool water's CH to 990? Did you add tons of calcium chloride pellets at some point?

Richard