The fewer times you pour and measure acid the better -
This is ho I do it
Safety first. Wear old clothes and eye protection.
I like to get in front of a return on my hands & knees and splash a little water on the pool deck.
Then open the acid bottle and partially submerge it in the water. Tilt the bottle to pour the acid in at close to water level, but don't let the bottle get submerged.
I don't measure because it's an additional step for me to spill the stuff.
Remove the bottle from the water and set it down in the puddle on the pool deck while you replace the cap. Try not to breathe the fumes..
Guess a little low on amount you need, then test again after it circulates well. You can always add more.