Looks like you may need to use several voltage levels, depending how far away the active zone is. Voltage level needed for 70m zone will probably not be enough for 250m zone while voltage level for 250m zone may fry the solenoid 70m away.
To solve your problem a transformer with multiple wingdings (outputs) may be needed, something like this (link). The good news is that you will be able to get rid of 500w 220v to 110v power converter.
What is your voltage level at the nearby zone one (15m one)?
Also what is your voltage level right at Rachio (as measured between SC and SP terminals)?
Do you have the 250m zone already setup? What is the voltage there?