Thank you for the links.
I'm still unsure what to do about the cement:sand:agg 1:3:3 ratio though, seeing as my agg is a blended mix and contains the sand already.…

I don't need to add sand cause the 19mm agg mix I'm buying already has sand blended in. All you need to do is add the cement powder and water.
Unfortunately…

Hi, first time building anything, first time posting here, so please forgive me.
I'm trying to calculate what I need to order for the concrete for my footing post holes.…