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All of us should be trying to get the greatest Noise Reduction Rating (NRR) that can be put together. NRR 30 plugs with NRR 20 muffs will give you an effective NRR 45 (add plugs and muffs, then subtract 5).
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Where did you get this NRR formula? What is the reasoning to add the levels of protection and then subtract 5?
Why would you use 30 db nrr plugs but only 20 db nrr muffs? Why aren't you using better muffs?
For the ease of simplicity, assume you had 30 db nrr plugs and 30 db nrr muffs. According to some folks, the combined use results in only a 33 db nrr. The reasoning is that since db pressure doubles with every 3 db increase, then with doubled equal protection, the additional db nrr reduction only changes by 3 db.
The math seems correct, but it is based on a single point barrier for reducing sound and this is not the case when you use multiple barriers such as plugs and muffs combined that provide a sequence of noise reduction.
For a given sound level, say 150 db, the 30 nrr muffs would reduce the noise to 120 db that gets past the muffs. The 120 db then encounters the second 30 db barrier and then the sound is reduced by another 30 db that leave you with 90 db going into the inner ear. With two barriers, the second barrier has no knowledge of how much sound (pressure) has already been reduced. In fact, it has no idea if another barrier was present. The second barrier, the plugs, simply work with the sound they encounter regardless of how that sound was manipulated on the way to the plugs.