The -12 thing is mostly important when tracking, and here the panning has no effect (that happens after the recording has taken place).
You are recording a mono track; how much of that track goes to each speaker is something that happens in the DAW, so you don't RECORD centered or RECORD panned - you pan it afterwards.
The reason the meter has a higher reading when panned is due to something called "pan law". Different daws have different versions of it, but basically it is to compensate for something; namely how the sound pressure from the speakers relate to the level set on the track fader.
IF you have a track panned center, you have sound coming equally from both speakers.
If you disconnected the right speaker, you basically have the same SOUND as before, but now there is only ONE speaker blowing it out. Thus, the sound is not as LOUD.
Pan laws basically turn down the sound as the panning moves from the far side to the center to compensate for this. It seems like your pan law is set to -6 dB in the center. Other "pan laws" have other magnitudes of this effect (typically +3 or +4.5)
There are three reasons to keep the recorded signal at -12, or possibly -18 or -24 if you're going to have a lot of tracks.
1) Avoid clipping: WHILE RECORDING, as this cannot be fixed later. shooting for -12 on the peaks should make sure that even if you strum harder during the recording, there's still headroom.
2) Mixing: Some plugins are NOT exactly designed to handle material that peaks at 0 (yes, you CAN clip a plugin depending on how it is made - it's just not as obvious when it happens, but it alters the sound, especially if several plugins and tracks do this)). When I mix a song, i play the whole thing through with all faders at zero, and see where each one peaks (with the master fader muted, so no sound), I then put a gain plugin AS THE ABSOLUTE FIRST PLUGIN IN THE CHAIN, and use that to turn down the level to -12 or -18, depending on number of tracks. But this is a concern for mixing, and it doesn't seem like you're at that point yet.
3) Summing. All tracks are "summed" on the master fader, so if you have 20 tracks that peak at -3 dB, then the master will probably clip (depending on the material), So this is another reason for keeping levels low - and lower if you are going to have many tracks.
I *THINK* the pan law is applied AFTER the plugins. So -12 is probably fine.
Otherwise, the thing to do is to shoot for -18 db on the peaks when setting levels, before recording, that way you should reach -12 db when panned far left and right (which you often do with guitars in a mix). That's what I would do, beacuse I'm anal about not getting too high levels. Remember to set the level while looking at the corresponding input strip of the focusrite Mix Control - NOT in the DAW.
-18 IS NOT TOO LOW (if you record in 24 bit depth). You compensate by turning up your monitors
A little story: In the olden days, they often calibrated their meters so that their "zero" were at the levels where the saturation of the analogue gear kicked in, so anything below 0 was a clear and clean signal, and above that was where the saturation kicked in as you turned up the fader or gain.
That 0 would often be around what is shown as -24 on our meters in the daw today. That's why -24 dB is NOT too low a level to record
(again: provided we use 24 bit depth resolution)