Sure LeadSled, the law is the law, however if you forgot to time a flight then you need to put something in. Logging less than your actual block time is illegal as is logging more, so the only thing to do is to put in a good faith guess. No one's going to take you to court for that as long as you're not making a habit of it.
The worst case I've heard of people using rule of thumb times was a company in NZ that was timing blocks off to blocks on and then
subtracting a set amount for taxi and coming up with a dodgy flight time (resulted in some spectacularly fast Cook Straight crossings
.) That is much worse than adding a set time to a measured flight time to obtain your log time.