Surely it's important in this type of discussion to remember that mental mathmatical gymnastics requires a person with the mental ability and agility more related to a selected and young RAF trainee, flying and 'living' aviation on a daily basis.
Most of us in general aviation may be as bright as them but are not necessarily as young or as regular in our practice of in-flight mathematics.
Nothing can replace the experience of our more mature years but there is little doubt that, for the hobby flyer, nothing can better detailed pre planning for any flight - anything which will reduce mental effort during the actual flight must contribute to safety.
In G.A. we have the luxury of a) time to pre plan and b) aircraft with lower speeds and less demands on the pilot than military aircraft, e.g. we do not have weapon systems to manage or low heights to negociate.