There is no direct way to deduce departure time from boarding time. Different airports, different countries and even different airlines have different policies on this, so there is no fixed "It will depart x minutes after boarding" that you can rely on.
What you would be better off doing is keeping a written itinerary if you are prone to forgetting. I do this even with small journeys, using TripIT so I have a copy in my phone, and a paper printout with me, both of which containing all my flight info, terminals etc, connection times, hotels booking numbers and meeting times.
As commented by @DJClayworth and @jwenting, other differences can come from the transit times from gate to aeroplane (is the plane at the stand adjacent to the gate, or is a bus required to take you to the other side of the airport?) and from logistic or technical delays of many kinds.