I have a softub 220. Its time for a refill after being empty for the summer and I had some additional questions. The TA of my tap water is around 375 and calcium is around 275. During initial balancing I have been using dry acid dissolved in water with a separate container. I would put no more than 1/2 a cup in and aerate for 30 minutes. It seems to take around 1 3/4 cups total before the ph and ta would be at a acceptable level. I don't have the exact numbers as its been a while since I filled it. This takes a very long time but I wonder if even 1/2 cup is to much at once? Whats the max I should be putting in and how long can I wait between doses? I would like to switch to the half strength muriatic acid since my ta is so high but was wondering if that would be safe on the vinyl if its mixed with more water?
My other question is what is the easiest way to figure out the exact amount of water? I'm thinking 220 is being filled to the brim and I just need to figure out how many gallons per inch so I can do a better estimate with how far the water is from the top. Other than that I'm thinking of buying a scale so I can do more precise measurements with the initial dichlor or any other powdered stuff I might need to use.