Determing Purchase Points for Map Size

[gRaider2001]

Hello all, first I would like to say that I really enjoy this website. I have not played a pbem game yet, but hopefully I will have the chance to play one soon:-)

The question that I have is how do you go about determining the number of purchase points to use based on the size of the map? Or, what size map is best for the number of purchase points that you use? For example, I have determined I like use 100-150 units. What size map would be good for that number of units, and how do I determine that number of purchase points that will allow me to buy that amount of units?

I understand a lot has to do with the time period and country. For my example above lets say it is based off of the U.S. during the 1960s.

Thank you very much for taking the time to read this question.