I have a shortcut to center outlines within the width but… it would be nice to have a metric key that maintains it automatically and so warns when the glyph is not centred anymore. Like I would enter # on both sides and boom, the value for a centred glyphs would be calculated automatically. It would spare a lot of time.
Or does it exist already? I haven’t find anything in the doc.
Do you mean =|?
No, this is just mirrored from the other side. If I change the length of my outline, then both side bearings have to change for the glyph to be centred in the same width as before. And I want to avoid that.
Or do you mean I just enter this on both side without value and it works?
I see. So you would like to keep the width the same (such as explicitly writing =600 for the width key), combined with =|, but without writing =600? That’s not currently possible.
I just tried the thing and it actually works without blocking the width! But one of the value is blue. But hey, never thought of trying this, that’s progress.
Edit: after re-starting, the =| goes back to a number value. So the fix would just be: allowing this situation 
Edit2: it’s blue because I am dumb and wrote it the wrong way.
=xxx in width and =| in the RSB. Leave the LSB without a key.
And the key is always =| (not |=)
\No but that is the thing, “fixing” the left side bearing is what I am trying to avoid. And as a matter of fact,\
when I write it correctly it works! So problem solved. Don’t mind me, Sebastian was right from the beginning
EDIT: I mis-read what you wrote Georg. ALL GOOD I got it now.
Updated image for my fellow primates, this is the trick:
The trick doesn’t seem to work in 3.5
the update buttons seem to be broken. But Glyphs > Update Metrics, does.
