At the most basic level, yes, heat loss is a linear function of the difference of the temperature inside vs outside multiplied by a constant which represents your house's thermal conductance. However, in practice this method of predicting heat loss is only a very rough estimate and only useful if dealing with day, week, or monthly averages using heating degree days. For instantaneous predictions things like sunlight (angle of incidence, intensity, and where it is hitting the house), cloud cover, wind, rain, humidity, snow, etc. can all wreck (often times severely) a perfectly good estimate based on the ideal, linear heat loss calculation.
If you're wanting to figure out what BTU rating a heating appliance needs to have for your needs there are books and worksheets available to guide you through the calculations. Yanche (or maybe is was LsFarm or WNY) has posted them previously, you should do a search for his name and maybe "heat loss calculation", "boiler sizing", "boiler rating", or other such things. Could also PM him for the links. Lastly, you could contact a local boiler dealer or HVAC contractor to have them do this measuring and calculation for you. This last option may be a good way to go if this is the first water based heating system installation in the house and/or you aren't particularly adept at plumbing work.