No probs.
Pre-
spray (sorry to be pedantic!) coverage varies according to the carpet construction, pile fibre, pile depth (remember it's a 3D object you're cleaning), product being used, technique being used..... soil conditions.... many things
A basic rule of thumb I work to is to apply enough to do the job but stop short of over-wetting. It's just something you get a feel for as you go along.
As an example, if I was using a portable system to clean an average sized through lounge (230ft²) which was a polypropylene pile, and quite grubby, I'd probably put down a good 4-5 litres. However the same sized carpet which was an 80/20 in good condition would probably only need 2-3 litres.
It's no good just tickling it, whatever carpet you're cleaning.. you have to be committed because fibres need wetting out to make your product work. But you need to know your limits, and have a feel for what is "enough" and what's "too much". The back of your hand is the most sensitive precision instrument you possess
I'd suggest when you get the opportunity, get yourself as many different carpet samples of different types as you can and wet them up progressively from one side to the other, so one side is lightly damp and the other is sodden. Brush them over and then after 5 minutes look at the back to see if it's wetted through. On some carpets you'll be amazed how little water in the pile is needed to wet the back.
But don't be under any illusion that no carpet cleaners get carpet wet to the backing... I'd even go out on a limb and suggest that after extraction a lot of carpets would be at least a bit damp on the back. But within acceptable limits, this moisture will not cause harm and will wick back up and evaporate in reasonable time.
Sorry for the ramble
