If it's actually 7.8, and not "at the top of my test kit scale which is 7.8" . . . you'd be OK. The problem is, if it's at the top of your kit's scale, then 7.8 REALLY means 7.8 or higher, not 7.8 actually.

However, given the high CH, you probably need to hold a pH of 7.2 for awhile in order to lower your TA, which drops when you hold your pH low AND aerate. (Lower pH turns some of the TA into carbon dioxide, when then leaves the pool slowly without aeration or more rapidly with it.)