Setting the INST option gives you a max input level of +21 dBu, @ 8 dB gain! So to get +13 dBu as max input level you need +16 dB gain.

nwarrick wrote:

I'm thinking it should be around -13 to -15 or so. Somewhere here the maths is wrong. -25 its even hard to see the waveform even zoomed. Seems far too low.  This doesn't add up.

SMPTE recommends -20 dBFS as the alignment level for 24 bit recording, EBU recommends -18 dBFS.

This article explains these considerations very well: https://www.soundonsound.com/sound-advi … -recording

I never made such math and simply oriented on getting a good input signal for
- recording and
- routing my guitar input signal further
because I have different use cases anyway...

I route my dry guitar signal (directly from the guitar) to my amps (overdrive pedal in front of it) and
record it for any potential reamping purposes or use it for VSTi.

Should I always use a different gain value for the guitar? I think no.

I think I should simply do what you always do in front of an A/D conversion.
Look that you get a good input signal, but keep enough headroom.

Here, RME's Autoset feature was of excellent help for me.

Enable Autoset on the Instr input, raise the gain to a value, that Autoset has something to do ... then strum harder and harder into the strings. Besides strumming into the strings use every guitar / playing technique from which you know that it produces high peaks like pumping rhythms / power chords on the lower E-A--D strings. Guitar volume and Tone pot at max (not dampening higher frequencies). Use each of the PUs single and also in combination, you never know whats louder.

At the end (after using Autoset for all guitars) it boils down for me that I use with UFX III
- 20 dB Instr Gain for all Humbucker guitars (all with Gibson '57 Classic PU)
- 22 dB Instr Gain for one Single Coil Guitar with Texas specials, which are a little bit hotter

Of course, since you don't always hit the guitar strings like a savage with extreme power, using the gain values determined by Autoset will ultimately result in input levels that are not too hot, which is what the SOS article quoted by unplugged also emphasises, i.e. not to overdrive when recording and mixing.

By this / using RME "Autoset" I am getting wonderful recording levels for all guitars.

This screenshot as an example playing power chords in a more normal way with absolute peaks of -8.8 and good RMS.
Part of signal flow in short: via Guitar--->InstInp9---UFX III--->Amp Input

EDIT:

If you should find out that the levels for playing over VSTi do not match, because there is too much or missing gain,
then I would try to find a way to correct that in the VSTi.
There should be something like a global knob to adjust the input volume.

Otherwise you would have instantly to switch Gain Levels on the Recording Interface which makes no sense to me.
Better one level there and adjust the VSTi, because then also reamping with different VSTi or VSTi presets will be
possible without not too much hassles.

SGear3, which I use, has something like this in the I/O panel, not sure whether this is per preset or a global setting

Maybe there are different ways and finallly it is important that it sounds ok. From operational perspective I like my approach to consequently use Autoset to find the proper level for all of my guitars very easily this way and to have a unique input level no matter which guitar I use.