Text or be hexed?

Proving that the last will at least be penultimate, The Shadow casts itself once again, and this time with one of the great dilemmas of our mobile age:

What degree of lateness requires an explanatory text message?

If you're five minutes late for an appointment do you have to say "sry im l8"? What about two minutes? Half a minute? Where do you draw the line?

So far I've looked at the time factor but does the type of appointment make a difference or not? Do you send an SMS when you're ten minutes late for a casual coffee, but also when you're ten seconds late for an important meeting?

What about who you're meeting up with? What about how you're getting there - surely if the other person knows you're on a city rail train you don't need to explain!

I think this is one for the great mathematicians of our time. Who can come up with the formula that reflects the relationship between all these factors.

But they'd better hurry. The solution to this problem is long overdue!