Hi Miaad,
When you assign the counter in the plan, it will calculate the current reading and save it.
Based on your OR or AND function system will determine whether the call can be made or not.
If you have maintained 3 Mon or 250 hr, your start date will be efficient to calculate from when the equipment is running and calculate the plan date accordingly.
In Short, you need to consider below points
1. Present Counter reading
2. OR or AND function
3. Start date
Regards
Terence