Just play the pink noise and adjust left and right speaker volume to 85 dB (or 79 dB in my small treated room)? There is a mild volume drop in the right speaker when compared to the left one. Should I use console fader and pan to match 79 dB level on boht left and right speaker?

If your console is calibrated to send the proper signal that it shows, then simply balance the INPUT SENSITIVITY of the right speaker to match the left speaker output.

If your console is not calibrated to send the proper signal levels Out then all bet are off, and you're guessing like most people do... It's a matter of 'who do you trust'. When you know the source is accurate (and let's hope your console is telling the truth) then adjust the receiving device (the speakers), it's that simple really.

If you are a real stickler for accuracy, hire someone (there are still licensed individuals out there) get it right. In most home studios, there is so much wrong from the standpoint of reflective surfaces, obstructions, glass in the wrong places, rugs, etc., etc., etc. What you will learn from the experience (hopefully) that when it sounds right it really is... And when it really is right, it does sound right! You may find that you only spend that money ONE TIME! It's very much like looking at a SPECTRUM ANALYZER - you know, the ones that tell you how much energy is in specific ranges of your mix... after looking at it and listening for a few weeks, you conclude that while it is interesting - it mostly proves to yourself, that when it sounds right it looks this way, and when it looks that way it might just sound right - but you wind up believing you ears (anyway!) in the end.

What is the lifespan of studio monitors? There have been many topics on this, some people say 15 years, some say forever. I noticed that recording studios still use NS10.. They are pretty old....

Nothing lasts forever... so that’s an exaggeration. Although I’m sure they mean figuratively speaking. The NS10s are still around because accuracy has staying power. Right combination of size, and sonic alignment for the purpose of critical near-field listening. That does not change.

The exaggeration comes from, in theory, as long as the magnets work the speaker should continue to function. “Permanent magnets” are as permanent as we as humans need to concern ourselves, since they were magnets when we arrived and they’ll likely be magnets long after we’re gone! Other components may eventually fail but not so you’d need to concern yourself.

Speakers that are trendy are typically for consumers who are easily impressed by hyped sound! Studio Monitors are designed to avoid hyping the sound. That’s why NS10s are found in so many studios to this very day!