My supplier switched bag sources three years back and the lime content changed without anyone telling us, that threw my ratios off way more than humidity ever did. I mean, I checked the bags and they looked identical but the chemistry was totally different, you could feel it in the mix before it even hit the trowel. What finally helped me was doing a quick slump test every morning before I started, just a little cone and some water, takes two minutes and saves you from ripping out a whole wall later. Maybe it's just me but I think the humidity thing hides a lot of other variables, like aggregate moisture sitting in the pile or even how long your wheelbarrow sat in the sun. That August morning you're talking about, I bet your sand was sweating too, not just the air. Get a cheap hygrometer and a moisture meter for your sand pile, that combo caught my cracks before they ever got going.