Normal
DNO version is add up the MCBs and multiply by 0.4, but it's all drivel, if I rewire a house, I use lots of circuits because more is better, and end up with figures that say the main cut out fuse woud last 10 seconds. At the risk of sounding rude, you're over thinking a non existent problem. Get a sparky in and get the shower fitted, I will be very surprised if anyone refused to do the work