The tried and tested method I use is the same as someone else above...
I add all of my times up in minutes and then divide by sixty. Take away the whole number (hours) and then muliple the remainder by 60 to give me minutes.
I usually enter my times the the nearest 5 minutes (give or take) so it works out pretty well.
Takes a bit of time, especially with all of the columns being used now!!!
Best wishes,
Charlie Zulu.