When you say 'the Pool calculator' I'm assuming you mean PoolMath here at TFP.
https://www.troublefreepool.com/calc.html
If your TA is at 70 ppm, just enter that in both the "Now" column and the "Target" column. TA at 70 ppm is fine. Later on, after you get comfortable with the pH management, you may choose to lower it a bit to reduce the frequency of acid adjustments, but that's entirely optional.
If you need to reduce TA because of your calcite saturation index (CSI) exceeding 0.3, then you can follow this procedure:
Pool School - Lower Total Alkalinity
If your pH is at 7.8, it is in range and can be left alone unless there is some other reason to adjust it. When it rises above 7.8, adjust it down to 7.6 or perhaps 7.5. If you take it any lower than that, it tends to rise back up to 7.5 or 7.6 very quickly (i.e. often the same day). For PoolMath to calculate the amount to add, the TA number in the "Now" column must be entered. The TA number in the "Target" column has no effect on the calculation of acid required.
At the bottom of PoolMath, there's a section called "Effect of adding chemicals" which will show you that adding e.g. 15 oz of dry acid (when TA is 70 ppm) will reduce pH by around 0.24 pH units, and TA will drop by around 4.2 ppm. You will not notice a change of 4.2 ppm on the TA test, because the error of the test is +/- 10 ppm.
Be sure when testing TA that you add drops until there is no further color change. Some people call the color full "Barbie pink".
Maintaining pH between 7.6 and 7.8 should be possible with weekly acid addition, or perhaps twice a week.