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I've mixed concrete before, but only as a labourer or kid when I had someone who knew what they were doing to say "more of this/that" or just "that looks about right".
I mixed tons of concrete for my dad as a teenager when we built a garage and concreted the driveway to it. His mate came to help once and mixed the concrete that day, the stuff he mixed degraded far quicker than the stuff I mixed. As @Colin_the_Bald says, getting the consistency right is key.
I've mixed concrete before, but only as a labourer or kid when I had someone who knew what they were doing to say "more of this/that" or just "that looks about right". I think my main concern would just be getting the mix wrong and having to pay more to get it lifted and relaid. I may go with the clearing and shuttering myself so then I can give more definite specs to a builder or local landscaper. I'll ask my builder if he does that kind of thing when he's next here.