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The central obstruction of a secondary is referred to as a percentage of diameter , usually. Not sure why the area percentage is not the more often quoted as you'd expect the area to be more relevant to affecting the contrast. Probably is a physical law pertaining to light waves that I'm not qualified to offer any insights into.
For planetary viewing or imaging, less than 25% is often quoted as desirable and >20% as an even better option. Even large SCTs' with central obstructions over 35% seem to be able to capture good detail though. A lot of great images posted to the ALPO site are taken with C11 and C14s'. Comes down to seeing conditions as much as anything else but an optimised scope in great seeing is going to make the most of the optics used compared to something with a larger central obstruction.
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