You could also put on your jumpsuit, vest, and girth belt. Make sure everything is where it will be when the complete costume is worn. Then draw a line on your vest with chalk or something easily removable along the top of your girth belt. Then remove everything and lay out the vest flat and find the center (2 marks one high and one low to keep the line from going off center) then draw your center line (with chalk) from the neck to the girth belt line. Use the center line to center your collar, diamond, and abe armor. Then it's just a matter of evenly spacing out your armor in the space allowed between the colar armor and the line you drew where the girth belt will sit.
Then as you stated I would still recommend temp installing it to see how it looks when worn before installing the snaps (just to make sure).