Although - referring back to the original topic title, you mean BIAB
In which case - there is the guide to help you and also a number of posts around the place where I have explained how to work out how much water you need.
I'll have another try -
you say you want 20L in your fermentor, so you work backwards from there.
You will lose 2-3 litres of your volume when you leave behind all the hops and hot break etc etc so to get 20 you need to add that = 22.5 - 22.5L is your batch size, when you are working out your recipes, remember to work thejm out for 22.5 litres because thats how much wort you are making. Not the amount you actually get to put in your fermentor.
You will be boiling for an hour, and you will probably lose around 4L per hour to evaporation. 22.5 + 4 = 26.5
Your grain will absorb liquid - about 0.75 L fro every kg of grain. Lets say you are using 5kg of grain. So thats 0.75x5= 3.75
So in order to end up with 20L in you fermentor and compensating for losses due to trub, break material, evaporation and absorption by your grain, you will need to start with 3.75+26.5 = 30.25L of water in your pot.
As you do a few brews, you will get actual figures to replace the approximates I have used in this calculation. Take lots of measurements when you brew and you will know whats happening at each step.
Make a calibrated dipstick - with graduations every litre so you can tell what volume you have in your kettle.
Measure cold water into your kettle, the 30.25L I mention above. Then heat it up to your strike water temp - probably around 68C - measure your volume again and note it down. It will have changed slightly because water expands when it gets hot.
Take volume measurements at the start of the boil, let it just boil - turn off the heat and measure the volume (also grab a 100ml sample to measure your pre-boil gravity, tip it back in during the boil) if you subtract this figure from the strike water volume figure you wrote down before, you will know how much liquid your grain absorbed and can work out a L/kg rate to use in the future.
After the boil has finished - another volume measurement. Subtract this from your kettle pre-boil volume and you will have your evaporation losses. You can use this to work out how many litres per hour you boil off.
Cool and settle your wort, transfer the clear wort to your fermentor. When you aren't able to get any more wort without transferring break material and hops, stop. Tip whats left in your kettle into a measuring jug - thats about how much you lose to trub and break material. It will change a little with each brew and I would give yourself half to one litre of leeway. Worst that can happen is you get an extra litre of beer in your fermentor.
In two or three brews time, you will start to get a handle on how your system behaves and what numbers you get at each stage, this will allow you to tweak your expectations and starting volumes, so that what comes out at teh end is what you want it to be.
Cheers
TB
Flattop - I'm pretty sure the guide doesn't suggest a nylon bag - it suggests a bag made from polyester swiss voile - which is, well.... polyester. 100%. Heat doesn't affect it and yes, Gryphon Brewing (site sponsor) sells them pre-made if you have a burning desire to spend real cash on something that costs about $5 if you make it yourself. I'm sorry to get picky on this, but there have been probably dozens of posts on making bags, buying bags, what bags can and cant be made out of; and what shape they should be.... this info is already there on AHB and a little research would answer these questions for you.