In answer to your question on mixing it by volume instead of weight.. Yes, you will go wrong, unless you can find any info on the weight of a litre of powder. I find trying to judge half a bag, or a quarter of a bag etc practically impossible once you start pouring. You'll want to start with fairly small amounts anyway for rapid set, as it goes off pretty quickly.
I mix it til it feels quite stiff (hard to mix by hand) then leave for a few minutes and mix again. If it's still really stiff you can add a little more water.
I mix it til it feels quite stiff (hard to mix by hand) then leave for a few minutes and mix again. If it's still really stiff you can add a little more water.