The “slightly formal” untucked shirt is what I almost always wear (outside of work). There are several styles, and I am not the best at describing them. First, we have the basic Hawaiian shirt (I have about 6 nice ones—take your time, find an expensive one on clearance—you can tell the good ones by the buttons and the quality of the material).
Then, there is a category of shirts, some with designs and others with subtle textures (which can look very dressy), and I am not sure what they are called—either “Cabana shirts” or “Cuban shirts” or “Guayabera shirts” or whatever. They vary from shirts with vertical lines, to designs, to a business style shirt; but what they all have in common is that they are designed to be worn untucked. I have about 10 of various styles.
These styles allow a middle-aged guy like me to properly wear an untucked shirt and still not look like a slob. I love them.
Second the open Hawaii shirt over a tucked tee, using IWB Sparks Summer Special, spare mag in a Buck knife leather pouch. Fits a single stack (1911) just fine, looks like I'm just carrying a folder when I expose it using my cell phone.
Of course the Hawaii shirt helps to prevent printing.
Good thing about better quality shirts is they're heavier, and since a good concealed draw is a Texas Two Step - sweep shirt back, then draw weapon - the heavier shirt has enough inertia to swing back out of the way.
A cheep, thin shirt just flops and gets in the way of the draw.
One thing about carrying... I moved my cell phone clip/holster from right side to left and got used to using the phone with my left hand. This is in case I ever find myself with a handful of 1911 and need to call 911 with the other hand.