Rule that ads value to current state

Hi I’m using openhab3 fully with the gui,

I’ve got a request for a simple rule:
i have a dimmer button that reports “brightness_move_up” on every ‘click’ when turning.
I’d like to ad 5 to the brightness of a light for every ‘click’ received.

triggers:
  - id: "1"
    configuration:
      itemName: ZigbeeDraagbareDimKnop_DraagbareDimmer1input
      state: brightness_move_up
    type: core.ItemStateUpdateTrigger
conditions:
  - inputs: {}
    id: "3"
    configuration:
      itemName: ShellySpot10shellybulbduo8caab56171f61921680222_Brightness
      state: "100"
      operator: <
    type: core.ItemStateCondition
actions:
  - inputs: {}
    id: "2"
    configuration:
      itemName: ShellySpot10shellybulbduo8caab56171f61921680222_Brightness
      command: "5"
    type: core.ItemCommandAction

the rule above just sends the command 5, any easy way to send +5 ?

No, you;'ll have to use a Script Action. For this Blocky should work or you can choose Rules DSL or JavaScript.