agreed maximum load always has been always will and should be..
Sorry, I just can't agree with this.
"Maximum load" results from a calculation that is full of assumptions and guesswork.
You use a figure for the u-value of a wall that is educated guesswork in the case of modern construction methods, and a rough approximation in the case of older properties.
You assume an outside temperature which would have been wildly pessismistic this year (at least where I live) and wildly optimistic last year. Incidentally, you make the same assumptions for house on a windswept hill in ***bria as we do here in a protected spot in the sunny south coast.
You assume a number of air changes per hour which is absolutely a stab in the dark.
You assume that all customers have identical lifestyles, hot water requirements and preferred room temperatures.
Then (at least in domestic work) you guess pipe-sizes, utterly ignore the effect of pipe runs and valve restrictions and even how the radiator theoretical outputs were calculated.
And after all those assumptions, guesses and approximations, "maximum load" has about as much mathematical rigour as my estimate of Pompey's chances in the FA cup next year.
At the very best, you are covering your bum against claims by being able to prove that your system was designed to best practice. That may be a worthwhile aim, but lets not pretend its engineering.