Be very careful with straight muriatic acid. It can easily burn you, especially your eyes.
The "acidity" (that being essentially negative alkalinity) of muriatic acid straight from the bottle is about 11,000 meq/L (-31,000 dKH).
So adding 1/11,000 of the water volume as this acid will drop alkalinity by 1 meq/L = 2.8 dKH.
Suppose you want to drop 13 gallons of new salt water by 6 dKH:
If you want a drop of 6 dKH, you'd add 1/5,100th of the water volume.
13 gallons ~ 49 L
1/5100 of 49 L = 9.6 mL
So I'd add 5 ml and stir well for a few minutes and see what alkalinity you get. Then dose again assuming it seems on the right track.
You'll need to aerate well after adding the acid to blow off the excess CO2 and bring up the pH.
The pH willd roop quite low initially, and a side benefit is there will be no deposits when using normal IO with this method, while there is with making ir normally.