adjusting low pH

[fullhouse]

my TA was 220, so i added muriatic acid and brought it down to 120
which also brought down the pH to a little less than the 6.8 shown on my test kit. i believe i need to add borax to raise the pH back up it was originally 7.8
I have an AG 21' x 48" vinyl liner. with a sand filter.
any idea about how much borax needs to be added to adjust pH up to the proper level
i checked the chlorine level and the 10 second reading which i think is the FC is 3 and the 2 minute reading which is the CC is 5 does that sound like what it should be for just opening pool.
i have not added the cya yet. i did put in a dichlor tab last night that will have some cya, i will go to store today and get cya. how much of that do you think i should add to get up to a safe level.