I would say not the low level ones. Leveling new teams from 1-60, doing many quests + instances does not result in a lot of Guild Rep. After doing 3-4 teams to 60 i was only at 22%.

It does seem to increase with the level - in outlands I am getting a lot more rep. I am sure someone could post a number by clearing an 85 normal dungeon and seeing how much it gives with the bonus.

The easiest way is to just set up your chat tab (or an extra tab) to show Guild Rep gains. It will display the number... and you can just do that math after that. Whatever that number is... multiply it by 5. Take the cap and divide by that number - then that's how many times you have to run it!