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Keep in mind, your 15% may not be 15%. Batch to batch variations in both glass and film (most major film manufacturers adhere to a 3% variance, but some cheaper companies will let anything slide) can make predicting overall VLT of the glass and film combination hard to predict.
Also, some companies will call something 15%, when really it's 20%. Some call it 20% and really isn't 26%! Some call something 35% and really it's 30%. There really isn't a set naming system out there, so you have to go by the published specs (separate from the film name) and hope that companies quality control follows the industry standard of 3% variance.
Let's not forget that not all meters work properly too! Low batteries can cause bad readings. If they haven't calibrated it recently. A thin layer of dust on the window can lower the reading by 2% to 4%.
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