We don't use rules of thumb for geothermal heat pump installations.
People install geothermal heat pumps to save the most money possible. If the system is sized and designed accurately, it will save thousands of dollars on heating and cooling bills. If designers use rules of thumb and guess, and hope they are getting it right, then most people will get cheated out of performance and bill savings. Some people will get cheated out of a little bit of savings, and some people will get cheated out of a lot. Using rules of thumb is not good enough.
Installing a geothermal heat pump is more difficult and costly than installing a fossil fuel furnace, so it's important for you to get all the savings you're expecting to get. Anyone who is going to guess, and depend on luck for the savings, should just save themselves the extra work and expense of installing a geothermal heat pump, and stick with a fossil fuel furnace.
Not convinced? Professors Jeffrey Spitler and James Cullin at the School of Mechanical and Aerospace Engineering at Oklahoma State University lay out the facts:
Misconceptions Regarding Design of Ground-source Heat Pump Systems
"The use of rules-of-thumb for [earth loop] design length remains common in practice, and often leads to oversized, expensive systems or undersized failures. In reality, there are no generally-applicable rules-of-thumb that cover the diverse range of buildings and ground heat exchanger scenarios. Procedures based on building and ground heat exchanger simulation, accompanied by measurement of ground thermal properties will lead to successful designs. Though these procedures are more time-consuming in the design phase, they are a necessary prerequisite to successful, efficient GSHP systems."