Basically, you scale everything up linearly, except for water. Some portion of your water is more or less a constant (e.g. the amount that will boil off, or the amount lost in in mash tun dead space and/or kettle dead space).
So, if you know how much total water you needed for a 5 gallon recipe...
- subtract the "constants"
- take the answer and scale it up for the new batch size
- re-add the constants
Or use brewing software.
ETA: Since you are setting up a new system, this will be a little trickier, because you'd subtract the old setup's constants and add the new setup's constants.