I've been thinking some more about the trowelling. It certainly doesn't need "finishing" with a trowel. It's just occurred to me that your floor may be porous. If so, you may find it sucks water from the mix and stops it from moving around properly. That's when it might need a bit of "persuasion" with trowel.
In a "sucking" situation, it'll go off fairly quickly... even before it's found its proper level. The roughness of your floor won't matter.
Tip it into puddles of around a square meter, then wait for them to join up. This is where "persuasion" may be necessary.
The reason why I can't give the definitive answer is that each job is different. Floor suction, as I've mentioned... and the thickness needed for the low spots. Don't mix the whole bag in one go... and be prepared to move quickly if necessary.
I'm guessing you have a fairly level floor, but with a rough finish. If so, you won't need a great thickness.