It's just simple sums, joined together to make a complicated bunch of sums, Mike!! Yes, I used the Electrician equation, of Amps per volt per meter, Cable CSA, Cable length, wattage, voltage drop - were the ingredients - then you apply diversity - example, the Cable size in MM for an Electric cooker!
- in reality, a 32amp RCCB - & a length of 10mm Cable, would usually do the trick! Unless it was a really long run of cable !!
Glad I retired from all that 8 years ago