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The volume to use is the final volume you are aiming for in the fermenter; yes the post boil volume. Lovibond -> SRM °L = (SRM + 0.76) ÷ 1.3546
My initial guess was the Campden tablet's SO2 was the culprit and hunting around for similar stories I found this on a home brew forum: http://www.homebrewtalk.com/showthread.php?t=267418 Observations in this thread would seem to fit with what you have experienced.
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