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.

If you are four stages below you can use 1/16th the power.