The problem i think with scaling that peice is the width of the chest and the width of the under-arm pieces.
if you look and the piece in PEP Des, you will see the the space between the under-arms is smaller then the width of the chest piece. there is also a support piece that goes there. if you use this length of this support piece and the width of your torso, then you can see if it will fit the width of you (under of arms(ribs)) then the torso will fit. what you could do is just print off only that support piece to use as a measure.
what i did what i measured the distance between my arms( across my chest) i only scaled it down i little bit (maybe 2 cm off the ht.) just so i could put my arms forward better and to make the under-arms fit i just made the support-piece wider to get it wide enough to fit my torso(under-arms).