It'll show you either amps or watts - for your purposes. You'd normally plug this direct to the mains and then run everything else from that. The current drawn will depend particulary on how hard the amps are working so it's a good idea to get someone to look at this at several key points throughout a concert.
Don't forget you can use two adjacent (correctly wired) sockets to give you 2 x 13 amps. The only caveat here is that if the stage and the FoH are on different mains rings then you make get an earth/ground loop when you interconnect them with your stage and FoH gear.
I'm not going to offer any advice on what the likely maximum current draw is as I don't know your gear. I'd tend to quite happy as long as the peak amps is never above about 11.5 - assuming all your cables are correctly rated.
Always switch on and off gear in a sequence rather than all at once - with amps last on and first off, again in sequence.
In terms of calculations:
Watts (electrical) is volts x amps
Amps is volts divided by resistance
Volts is amps times resistance
Resistance is volts divided by amps.
So a 13 amp socket delivers 13 x 240 = 3120 watts (electrical not Audio "RMS"!)