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Hold the front of the mirror in front of a white light source then look at the back of the mirror and see how many "holes" and how much light is coming through the coating substrate itself. What it is reflecting is inversely proportional to what's coming through.
As a general heads up, the coatings on a Newtonian mirror can be in pretty poor shape, yet the telescope will still perform ok. Herschells speculum mirror would have been lucky to have 50% reflectivity, yet he was still able to find a few things with it. While there are a number of small marks on your mirror it doesn't look too bad. My 18" mirror has no coatings left on the outside 2cm of its entire circumference and it still performs fine. It's always set up beside 4 or 5 other 18" Obsessions and you can't really tell any difference. You might if you were pushing it on dim threshold targets, but you can't on the average everyday eye candy. Unfortunately that's what happens to 15 yr old coatings when you live near the ocean.
I would think the scope will be fine to use.
Cheers
John B
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