I do it the long way, which is to calculate the area of every surface (including measuring windows and doors), then applying the appropriate factor for the surface material and the temperature difference. Then I add the losses due to air change (volume of room x temperature difference x air changes per hour x factor for air). That gives the heat loss and you can choose the right rad. Often substantially different from the rules of thumb. You can build in refinements such as adjusting the U values for different severity of exposure. It takes about 1 page of A4 paper per room. The tables that the radiator manufacturers produce have to be read with some care, because they may be for a different setup than yours, for example they may be for one pipe connected to the bottom of the rad and the other at the other end at the top (BTOE or 'bottom top opposite end', if I remember correctly) but you probably want BBOE (bottom bottom opposite end), which conventionally requires a rad with 10% more output. You should also check the flow and return temperatures quoted, as condensing boilers should be run at lower temperature for max efficiency.
The 'Which' guide to plumbing & central heating contains worked examples and lists the more common U values. You would be able to borrow it from a library, if you don't want to buy a copy.
You can rework your figures for different assumptions e.g. the room adjacent/above is either heated when the room under consideration is , or it isn't.
