I think I need to do the 1.5" again ... 1.5 + 1.5, sewn together is actually 2.5" (not three) ... so, then you would cut a strip that is 2.5 + 1.5 which equals ... 4" unfinished strip for the borders ...
the complete strip you need to have for the border strip and the square would be: .... 4 + 1.5" = 5.5" ....
I think I may be making this too hard ... but, because of the partial seaming ... it still works out to 5.5 + 1.5 = 6 inch finished (6.5" unfinished) ... <wave>