If this is your thing a "rule of thumb book" is produced by Flight Time Publications
www.flight-time.com Itīs a pocket sized book of about 20 pages if I remember correctly. The latest price on the site is $7.95. Gives all the usual rules and some which are quite exotic, how you are supposed to work these out in your head I donīt know.
The rule of thumb methods are good, just to confirm that what you are doing is reasonable, itīs a question of finding out which ones suit you.
For those of you who only use the GPS, the new 1 in 60 rule is -
1 time in 60 the GPS will fail, due to batteries, not updated software, incorrect data put in etc etc etc.
PS I love my GPS
WH