You are correct about the TA level when you have CYA, but you won't have to change that manually. When you increase your CYA the TA will increase automatically by about a third of the CYA level. So if you get down to a TA of 60 and then later change your CYA from 30 to 60, your TA should rise to 70 automatically. The recommendation of not going below a TA of 80 was for a CYA level of 70-80. It's not a "hard" limit, but more of a judgement call to ensure still having enough TA in case of something getting into the pool that could lower the pH.

Richard