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When diving doubles you will need to use your wing/BCD to compensation. Typically you can't control your buoyancy only with a drysuit with steel doubles and stage bottles so you will need to use your wing to offset buoyancy.
On a chainring with 4 bolts the BCD is the distance between two bolts across from each other. It can be somewhat difficult to make this measurement if you don't have a caliper since the crankarm gets in the way. The image below shows this measurement on a 104mm BCD chainring.