Don't ask me where the 49.33 comes from, I don't know. In any case, I have seen other figures used, e.g 49.83.
It maybe a bit more accurate, but the heatloss calculation contains a total of up to 20% in allowances if I remember correctly, and my figure of 30% will not be far off.
Even if it is far less than I think, it is still 20 + 20, and never allowing for drafty doors and windows.
I think I will stick with my own less scientific system: quick look around, compare with another thousand rooms I have seen; K2 1400 x 600 will do nicely.
"By the way mr Punter, I tend to oversize the rads quite a bit so you could save a a score by using a smaller one, but it will take a lot longer to warm up nicely and if it is really cold, it might not be enough."
Have yet to find the first client who wants to save the 20 squid.
Radiator calculations are really a thing from the past.
Oversize them a fair bit, the room will heat up quicker, and the boiler will condense better. When it is warm enough, the electronics will turn the whole lot off so it actually works out cheaper to run.