Thank you for sharing that, Aymara...
Due to your much appreciated link, I've learned of yet another way to measure neck relief!
Here is another link -
I am now aware of four (!) methods to check neck relief. They all have one thing in common: fretting both E strings at the 1st fret.
Otherwise, well here we go...
1. Depress strings on the neck, beyond the 22nd fret. Check gap at 12th fret.
2. Same as 1., but check gap at the 8th fret.
3. According to a final inspector at Memphis, depress strings at 15th fret and check gap at 7th fret.
4. In the Gibson User Manual (2012), it says to depress strings at 14th fret and check gap at 9th fret.
I use the 4th method - but for kicks I then use the other three, to see differences in gap.