How to set trigger/item to increase for zabbix 2.2.6?

Question

Monitoring

I monitor a huge number of printers via SNMPv1
1. There is an item for the volume
2. There is an item for the current toner level
3. There is a calculated item to show the level as a percentage
4. We need an item that will add "1" to its value every time the punt value 3 is increased by 40 (approximately the number in the toner difference between the old and new cartridge)
Can you tell me?