No, no rule of thumb. It depends on you: how you wear your clothes (tight or looser), what
type of clothing you wear (casual or dressy), how fat or thin you are, how tall you /are. All these variables change how a given gun will conceal for
you. That's why everyone keeps saying it's subjective.
But everyone also seems to agree that the butt length and the width are more important in concealment than beginners realize.
Someone started a thread over at The Firing Line recently about choosing the carry method first and the gun second. It's an interesting thought. I really think you have to consider the two together - so many shooters only give consideration to the size of the gun, how many it carries, and what caliber bulllet it launches. Lord knows I've done that. But how are you going to
conceal that 4506-1 (it can be done, I've done it)? And are you going to be happy and comfortable and relaxed while doing it (after some reflection, I decided I wasn't)? Or how about the Glock 36 in my SmartCarry holster? I'm a lefty, and I found that the magazine release was situated in such a way that it would often be inadvertently tripped, dropping my magazine out of lock within the pistol butt while still in the holster. (I figured out a way around this, but it was a problem that NEVER would have suggested itself before I actually carried the gun in this way.)
The nature of concealed carry is that it takes a while (at least a week of packing, perhaps a month) for any given carry method/handgun combination to make its benefits and its flaws evident to the user. So folks go through a lot of them (we've all heard the old and true saw about the box of unused holsters that every concealed carrier has in his closet). There's nothing wrong with that, because there's nothing wrong with trying to find a system that works better for you. Ideally, we'd all have systems that would allow us to carry reasonably powerful weapons with decent firepower all the time.
And it will by necessity evolve over time. When I was a thin college student in Indiana, an IWB 4" K-frame .357 worked for me. I had a permit, and it was legal to carry anywhere I went: if it printed, who cared? Then I moved to a state with no provision for legal CCW off one's property . . . if I
had carried concealed, something else would have been in order. I got pretty chunky for a while. That would have affected what I could carry, and how (no more IWB for the fat guy!). Now, I'm a lawyer (thin again - thanks Dr. Atkins
) with a CCW permit, and I dress a lot differently than I did when I was in college. Again, different guns and carry modes are required.
Anyway, short answer is there's no rule of thumb. You've got to try it to see if it'll work. Best of luck with it.