What are the differences in stay duration between double entry and multiple entry visas for Thailand?

Um - help.

What is the actual difference days/months allowed in Thailand between multiple and double entry visa?

Double entry: enter January 1 extend February 28, exit March 31. Return April 1, extend May 31, exit June 30.

(6 Months)

Multiple entry: enter January 1 extend February 28, exit March 31. Return April 1, extend May 31, exit June 30.

Return July 1, extend August 30 exit September 30

(9 months)

But does the Third and final of the multiple entry have to happen within 6 months?

Effectively making the Multiple Entry a double entry plus one entry?

Please keep in mind the goal is to stay in Thailand and much as possible

The above scenario is an approximate days of exit entry etc meant for demonstrations only - no need to for the peanut gallery to go nutz