Using ph down

I'm going to try and save myself a bunch of work this year and start watering my plants with pond water once they get a bit older. I tested the pond water and the ph is 8. I went and bought a pint of pondcare ph down and I'm wondering how much ph down will be needed to lower the ph of a gallon of water from 8 to about 6.5. Any suggestions would be appreciated. Also, I'm using foxfarm nutes, would the nutes be acidic enough to lower the ph to an acceptable level? Would I only have to use the ph down when I'm not adding ferts to the water? I know that i can just guess and check but i'm just trying to get an idea of where to start. After adding the ph down do i need to let the water sit for a while before testing it?

OOps, the pond water has a ph of 7.5, my unsoftened well water has a ph of 8. I took a 17 oz sample of the pond water and added 5 drops of ph down to it and after about 12 hours the ph test showed just under 7. I guess I'll try adding 40 drops to a gallon of the pond water and see what happens... Anyone else out there watering with pond water???

I water plants with stream water. I have one of the cheap PH testing kits with the vial and the dropper bottle. The creek water is PH 8+. If I'm adding fertilizers to the water, I do that and stir it.. I usually just go by cap fulls. I dont use a dropper or anything. Just use the lid of the ph down bottle. I find that if I fill a 5 gallon bucket 1/2 full (2.5g) then it takes close to 1.5 caps to bring it down to 6.5 Depends on if Im using straight water or adding anything to the water. Just add a little and test, and add more PH down if required. If you over-do it and PH down too far, just add more of the pond water to bring it back up.

Edit: Somehow missed 1/2 your questions.

I am also using Fox Farm ferts and I have to use PH down for both straight watering and watering with ferts. You don't need to let it stand at all after mixing or PH'ing the water. Just mix your stuff, PH it, and away you go. Try your best to hit 6.5 but as long as you are close it's all good. Thats why there is an acceptable range.

Thanks for your reply. I'm glad to hear that I don't have to let the water sit after adding the ph down, that's going to save a lot of hassle. Hopefully that's all I have to do to the pond water to make it usable, hauling jugs through the marsh is a pain.