Ok, you have 3 knobs. Looks easy. TREBLE, MID, BASS. It is a no brainer. Or not?
I was reading an article on the web. The writer suggested to start with a setting. 3 numbers. No mention what amp or channel was used.
As you may know, reading about amps is a passion of mine.
Reading the above article i decided that there was some information that need to be shared with you reader. If you have a fender mustang you have up to 17 amp models (v2 numbers). So you need to know that every amp model may use its EQ in a different way.
Let me explain.
EQ can act in 3 different way.
1) 100% (all cranked) is the standard volume and 0% is (almost) no volume: the knob act as cutter
2) 50% (mid position) is the standard volume and you can either cut or boost the frequencies
3) 0% (opposite of cranked) is the standard volume and you can only boost frequencies (usually you can boos A LOT)
This 3 way are often mixed, so you can have an amp with BASS and TREBLE that act as a cutter and MID that can either cut or boost. So you can get totally different sounds in different amp with the same "numbers". You can understand the issue in the above article...
A concrete example:
my Champ 12 has no MID knob, only a boost switch, so you have only 2 MID settings, TREBLE and BASS are cutter, you can handle this but not every combination is possible. You cannot scoop mids!
American 90 model (like the good old mesa) higly boost Basses so you need to set the BASS know as much as it can but without became muddy.
Last tip: the tonal variaton of the guitar pickup conbination (Fender Stratocaster vs Gibson Les Paul) is about a +10%/-10%. Do your math!