Great question!

Since there is no percentage limitation on the amount of decrease that may be made under clause FAR 52.216-4(c)(4), I assume your question is one of the following two: First, whether you can go above the 10% limitation. Or second, if you must provide a limitation at all (10% or otherwise) when using an agency approved clause as opposed to FAR 52.216-4. I will answer both below:

Question #1: Can you go above the 10% limitation set forth in FAR 52.216-4? The short answer is Yes, the clause itself states "The clause may be modified by increasing the 10-percent limit on aggregate increases specified in paragraph (c)(4), upon approval by the chief of the contracting office."

Question #2: Do you have to provide a limitation at all (10% or otherwise) when using an agency approved clause as opposed to FAR 52.216-4? The short answer is yes, however there's no clear cut answer on what that limitation should be.  The intent of the clause FAR 52.216-4 is the share the risk of future price fluctuations, limiting the risk to both parties. Knowing that a limitation above 10% is allowed (see answer above) I recommend determining an appropriate (fair and reasonable) limitation cap percentage that does not over burden either party.