The propane company probably just wants some advance notice so that you don't run out. Depends on how big the heater is, but you should be able to run ok down to 5%.
Hi DZM,
Your regulator should be good as is. It has an adjustable output from 7-11" wc but should come preset to 10"wc. You would not want to reduce the pressure any further as that would cause problems with the burner efficiency.