Hi, My name is Philip. I am an attorney with over 16 years experience. Hopefully I can help you with your legal question
I am very sorry to have to bear bad news.
It is calculated at the rank you are discharged at.
The "high 3 rule" will not apply for a person who is reduced in rank due to misconduct.
You can see the rule herehttp://www.law.cornell.edu/uscode/text/10/1407
The key language is
(f) Exception for Enlisted Members Reduced in Grade and Officers
Who Do Not Serve Satisfactorily in Highest Grade Held.—
(1) Computation based on pre-high-three rules.— In the case of a member or former member described in paragraph (2), the retired pay base or retainer pay base is determined under section 1406 of this title in the same manner as if the member or former member first became a member of a uniformed service before September 8, 1980.
(2) Affected members.— A member or former member referred to in paragraph (1) is a member or former member who by reason of conduct occurring after October 30, 2000—
(A) in the case of a member retired in an enlisted grade or transferred to the Fleet Reserve
or Fleet Marine Corps Reserve, was at any time reduced in grade as the result of a court-martial sentence, nonjudicial punishment, or an administrative action, unless the member was subsequently promoted to a higher enlisted grade or appointed to a commissioned or warrant grade;
So if you go from E6 to E5, you retire at E5 pay.
Please let me know if you have more questions...happy to assist if I can