Yeah, it's been answered well and truly!! Can't believe I'm wasting time writing here...
All those with "rules of thumb" etc, repeat after me:
"Rules of thumb for logging of flight time are for JOKERS - people who can't be bothered looking at a timepiece twice in a flight, or doing sh!t properly in general..." 
Got it? It is about the easiest part of your job!!
So
why am I wasting time writing this post? Someone mentioned
Ops manuals, and I was reminded of a previous job where a total peanut CP (we've all had 'em!
)
insisted that I use
his rule of thumb for logging flight time
I said "ping off" and he started crapping on about getting it inserted into the Ops Manual if neccessary! Ha ha good times
CR.