Your method is a fairly reliable one.  I believe the rule of thumb is 1% of rated fire pump capacity though(flow, not pressure).

Given this, for a 500 gpm rated pump, you would size your pressure maintenance pump with ~5 gpm capacity.  

The jockey pump should never have a capacity greater than the flow of one sprinkler head.  

In terms of pressure, NFPA 20 2010 4.25.3 states "Pressure Maintenance pumps shall have discharge pressure sufficient to maintain the desired fire protection system pressure."

Here's a rule of thumb I use for setting controller pressure points:

Fire Pump stop point is churn pressure + city static
Jockey start is Fire Pump Stop - 10 psi
Fire Pump Start is Jockey start - 5 psi

So, your jockey pump has to at least be able to maintain the fire pump stop pressure.

 

Ok thanks for that information, but how about the underground leakage, does we need to consider this situation?