Where to store some configuration settings

I have a number of thermostats grouped together like this:

Group    All_Heaters           "All Heaters"                    <heating>
Number   Bedroom_Heater        "Bedroom Heater [%.1f °C]"       <heating>  (All_Heaters)  {zwave="13:0" }
Number   Living_Room_Heater_1  "Living Room Heater 1 [%.1f °C]" <heating>  (All_Heaters)  {zwave="9:0" }
Number   Living_Room_Heater_2  "Living Room Heater 2 [%.1f °C]" <heating>  (All_Heaters)  {zwave="10:0" }
Number   Hall_Heater           "Hall Heater [%.1f °C]"          <heating>  (All_Heaters)  {zwave="11:0" }
Number   Bathroom_Heater       "Bathroom Heater [%.1f °C]"      <heating>  (All_Heaters)  {zwave="14:0" }

Then I have the following in my sitemap to turn them all on/off.

Switch item=All_Heaters mappings=[15="All Off",22="All On"]

This works fine, but I don’t like the fact that I have numbers 15 and 22 hardcoded in my sitemap. This means that if I want to change them, I have to edit that file.
I would like to have two more Number items defined which will be called Heaters_Temp_On and Heaters_Temp_Off which will hold these two values. This way I can also add them in a sitemap and change their values with the mobile app. However, I don’t know how to use them in the sitemap. Do I need to write rules for this? Do you have any better idea?

Here’s one way (not tested):

Switch Control_All_Heaters "All Heaters" <heating>
Number Setpoint_Low "Setpoint Low [%.1f °C]" <heating>
Number Setpoint_High "Setpoint High [%.1f °C]" <heating>

add those items to the sitemap using widgets that let you control the setpoints.

rule "Toggle Heaters"
  Item Control_All_Heaters received command
  All_Heaters.sendCommand(if (receivedCommand == OFF) Sepoint_Low.state else Setpoint_High.state)

OK. So I do have to use code.