I apologize if my post sounds patronizing. It is in no way meant to be. Just trying to help...
A decibel (db) is a measure of sound. An SPL meter tells you how many decibels it's picking up. Turn it on and talk, it should respond. See
here for examples of how loud certain things are.
So with that in mind, the point of calibrating a system is so that all of your speakers produce the same volume. The 'set your SPL meter to 70-80db' is specific only to those of us with the old-style radio shack meters that are so popular. They look like this:
See the dial? That sets the range that the needle would register. It's set to "120". If the needle is pointing to "0" then you're hearing 120db (loud!). If it's -2, you're hearing 118db. +4 = 124db. But again, none of that really matters to you because you've got a nice digital one.
Imagine a thermometer. In your mind, you can see that it measures from -20º to perhaps 45º C. Now imagine a thermometer with a little window that only shows you 10º at a time. It'd be up to you to move the little window to the proper range to get the reading. If it's about 25º outside, you'd move the window to view 20º - 30º. The instructions you're reading are for an SPL meter that requires a 'window'.
But with your digital one, you've still got a 'window', it's just bigger. As JohnK says, it looks like 30-100dB and 60-130dB. It actually doesn't matter which you use because either range will give you readings in the 70-90db range that is usually used to calibrate speakers.
What you're trying to do with the SPL meter is to get all of your speaker outputs calibrated to play at the same volume given the same input signal. If they're out-of-balance, surround sound effects aren't nearly as convincing and your system just won't sound as good as it can. If your right rear speaker is closer to your seating position than the left rear, you're going to want to decrease the output from the RR and/or increase it to the LR so that they sound the same. By using an SPL meter, you can precisely dial-in the amount of +/- that you need. It's no different with the subwoofer, except that there are more variables involved with sub loudness because your sub has it's own amp & level adjustments. In other words, a sub's output can be quite 'wrong' compared to the rest of your system.
If all of your other speakers are pumping out 80db worth of test tone at X% volume, then you're going to want your sub pumping out 80db too*. The SPL meter just gives you a highly accurate way of making these adjustments.
Good luck! I'm sure there will be many others along to offer their assistance. We're all happy to help.
*Some folks (myself included) like to run their sub a little 'hot'. Mine is set about 3-4db louder than the rest of my speakers. I just like it that way.
