How much...

It varies from make to make. Mine is a 50 egg maino auto and I use one of those trays you get four chicken legs ( I should say that very quietly) in from the supermarket, and fill it. It will use about half of it over a couple of days, then it needs topping up cos it's getting too low. But I think some only take a cupful.
If no better help turns up start with a little then add more, and give it time too to go up before adding more each time. Mine has a fan and goes up pretty fast, but still air ones take longer before the humidity gets mixed round so to speak.

Thats what I got, a still air one.

It's got a tray in the bottom that has a sponge aswell, i've put about 30ml luke warm water in at the mo, and it's gone up to 50% so I will check again in the morning and add some more.

i am right to put kitchen towel in under the eggs today rather than wait til nearer the time? Trying not to open it so much now.

I put my towel under as soon as I stop turning for the same reason. Using warm water gives a faster change in humidity in my experience, so good idea. Try to measure just above the eggs if you can. I'm not sure but I think something I read said that still air incis can vary at top and bottom?
And from sad experience put some mesh over or rolled up plastic in the water tray if its shallow. Mines quite deep, taller than the chicks by a good bit so I didnt bother. Some how in the scrum of six hatching at once they managed to shove one over the edge and it drowned. I was so upset cos it was my fault for not covering all possible options.
By the way when they hatch leave them in the inci for a day, if you are using a seperate brooder. Gives them chance to fluff up, and get their legs working right. Also their cheaping 'eggs' (sorry) the unhatched ones on a bit.
Ooo, not long now..how are the fingernails doing?

That's the one good thing about this incy, the water tray sits under the egg tray, so the chicks can't get get anywhere near the water so I think i'm safe there.

I'm really struggling getting the humidity up tho, it's sitting at 55% and that's with the H meter sitting on the tray with the eggs, I did have it positioned over the eggs, but I felt it might have been too close to the heat source so had moved it. I can try and put it back where it was but have a feeling that isn't going to help so much.

I had read that you are supposed to keep them inside the incy for 24 hours after hatching, do you think I should get the brooder up and running before hand or wait for the first chick to hatch before doing so?

As for finger nails, I don't bite them as a rule but I am feeling the tension. I can tell it's playing on my mind, as I dreamt last night that they all hatched into fish and died.
They were pretty coloured fish at least, but no chickie's!

Gonna try and take my mind off it today and have a driving lesson (heaven forbid), I'm just trying to find ways to keep me away from the incy.