So, if the chest is 407mm and I increase it to say - 427mm wide, then do I have to increase the back and chest brace by the same factor (increase both by 20mm)?
thats right, just copy and paste the scale factor from one piece to the next and your the computer will do the rest for you. This however only works when the unfold is by the same person, if you say had the chest plate by bob and the backplate by joe then it will not work because they would annoyingly have set it to a different scale from each other as every person is different.