OH well, you've got the magical ingredient (the marine bio-spira), so you're all set
I'd let it run a day or two before you get the puffers to tinker with the salinity & make sure the salt is fully dissolved - & then you're good to go to add your puffers. I'm so happy for you!
And a lot of instructions say to aerate your salted water for a day or so before you add it to the tank (for water changes & replacement water) but I don't do that...I mix it
REALLY thoroughly with a plastic slotted spoon and make sure the water is clear before I plop it in the tank...but it only takes like 2 minutes. The Instant Ocean dissolves really quickly. I fill up the bucket - slowly sprinkle in the salt, then stir and stir and stir. But when you add it to your tank for the first time fill up, I'd let it run for a day to make sure it's all fully dissolved.