Lowering earth impedance--Earth spikes in Russ Andrews Book

My friend pass me an ancient copy of the Russ Andrew book--Power and Glory .
There is a short paragraph on placement of 4 earth spikes that are linked to a central earth spike .
My friend did the earth spikes in his garden and after hearing what it does to his system, i want to do the same.
I hope others with similiar experience can share their construction tips
What is the optimum distance between the spikes?
Is it possible to measure earth impedance with an ordinary DMM?
Can i have have 2 separate earth points or is it better to have an earth point dedicated to hi fi ?
I will engage a qualified electrician before i start the project .

thanks

kp93300