Equipment Levels are same as Human Power Levels
You can wield the full power of any equipment at your level.
A Master Level Equipment will require you to be at the Master Level to fully wield its power.
If you are at Expert, you can only wield half the power of a Master Level Equipment.
At Novice, you can only wield 1/4th the power of a Master Level Equipment.
Thus , if you are one stage below you can wield half the power.
If you are two stages below you can use 1/4th the power.
If you are three stages below you can 1/8th the power.