You were right about the $5,000/5% cap. I looked up the Internal Revenue Code for this topic. It's covered under Title 26 U.S. Code § 2041 - Powers of Appointment. The holder of a General Power of Appointment is treated for estate tax purposes as if he or she is the owner of the property. Therefore any trust assets for which a decedent is deemed to have had a "General Power of Appointment" will be included in their gross estate for federal estate tax purposes. The objective of a Credit Shelter trust is to avoid this.
There are a few exceptions specified in § 2041 that can AVOID having a General Power of Appointment and therefore AVOID the trust assets being included in the estate of the surviving spouse, including:
(b)(1)(A) having distributions limited to the "health, education, support, or maintenance" of the decedent or
(b)(2) having distributions limited to the greater of "$5,000 or 5%" of the assets (commonly referred to as the "Five or Five Power").
So either language in the credit shelter trust will work such that the assets will not be included in the estate of the surviving spouse for federal estate tax purposes.