you have already sewn your muslin and find the upper back is tight.
what to do? messing around with the armscye is asking for problems because you'll need to adjust the sleeve and in my book any sleeve adjustment,length excepted, spells trouble.
this adjustment keeps the armscye intact while adding some extra room to the back.
mark a line more or less halfway in the armscye to the center back then cut taking care not to cut through the armscye.
spread the pattern 2-3 cms.
put a piece of paper under the slash and tape it to the pattern.
there you have it! and no fussing with the sleeve.
you can now make another muslin or ignore this step and cut your fabric.
i do recommend as much as i dislike it, making another muslin.