At our matches, we consider the buttplate centered when the middle of the plate is not above or below the midpoint of the butt, it's not canted and isn't offset either right or left.
If the rifle comes with an angled buttplate -- providing cast -- that is accepted as normal.
The aluminum stocks have made the determination of 'center' somewhat subjective...some of them are marked without much real reference to center...and the 'bottom line' of the stock is non-existent. Deciding where center lies with them is often just common sense.
Frankly, if I'm running tech and the buttplate is anywhere near a reasonable center, I allow it -- IMO anyone who thinks that a small deviation is going to give him a serious advantage and wants to fudge the settings probably isn't someone who's likely to be in the hunt for top spots anyway. (Note that the keyword here is 'small'.) If it's markedly offset, I tell the competitor to either reset it or think of something else to do while the rest of us are shooting. :wink:
