(meteorobs) Question about fireball brightness

[Ed Cannon]

Several nights ago a friend and I saw a very bright fireball.
(I did submit a fireball report but probably can't persuade
my friend to do one also.)

This question is seeking a sort of rule of thumb.  I was
looking north, and the sky lit up behind me and I turned
around fast enough to see the last half of the fireball,
which was bright green and fragmented into a few pieces.
The question is, how much brighter than the local limiting
magnitude does a fireball have to be to attract your
attention from behind?  That night where I was (a suburban
location where on the very best moonless nights the Milky
Way is faintly visible), the LM was probably about +4.5.
My friend, who was not wearing his eyeglasses, estimated
it was -6, but I thought it had to be brighter than that
to attract my attention from behind.  So, is there a rule
of thumb about a fireball's magnitude in this type of
situation -- the fireball being bright enough (not to
mention lasting long enough) to induce you to turn around
180 degrees to see it?

Ed Cannon - Austin, Texas, USA