My rule of thumb is to service my bike and do as many repairs as possible with the tools in my bumbag , that way you know jobs you can do on the trail are possible with what you carry. I got a Bahco mini socket set that is awesome , I picked out the best bits to take ( sockets, torx , Allen key bits etc ).
I only carry 1 21″ standard tube on a trail ride , a spark plug , cable repair kit , spare chain links , quick metal and I have clutch , brake and gear lever cable tied to the bike under the seat.
My backpack has a small bag of nuts and bolts , tyre levers , pain killers , a compression bandage and snakes 
Hope some of this helps mate !!
Nick