Old timer at the ready mix plant told me to wet the subgrade before a pour
He said just a light spray would keep the ground from sucking the moisture out of the slab, but I hit it too hard and ended up with a muddy mess that took three extra guys to screed level, so what do you guys use to gauge how much water is enough on a hot day?
Learned this the hard way myself, flooded a driveway subgrade once and had to wait two extra days for it to dry out enough to pour. @miabennett's handful test is actually what I use now, grab some soil and squeeze, if it holds together without dripping you're good. Maybe it's just me but I still end up second guessing myself and running a light mist over it again before the truck shows up.
Read an old concrete book from the 60s that said to wet the subgrade until it stops drinking, then back off a little... that's the sweet spot. On a hot day you gotta mist it and wait a few minutes, not blast it like a fire hose. A buddy of mine swears by checking if a handful of soil crumbles but doesn't drip water when squeezed.