Anywhere from a 1 2 to a 1 overhang is acceptable as you can tell from the varying comments amongst the previous posters.
How much should a roof overhang a gutter.
Here phil explains how much overhang a roof should have at the gutter.
Metal is usually installed on rafters or strip sheathing rather than the solid decking used under other materials.
In my opinion the shingles should never be exactly even with the edge of any drip edge flashing.
You can add 3 to 5 inches to the top.
The reason for this is that whenever rain runs down any type of roof when it gets to the lower edge it clings to the tile or slate by capilliary action this means that the water actually runs around the bottom edge and slightlly back up the rear of the tile before gravity allows it to drop into the gutter.
The bottom tile or slate of the roof covering should project at least half way into the gutter the ideal distance is between half way and two thirds of the way over.
This will keep rain away from your shed.
A shed overhang is an easy and effective way to protect your shed.
To create a cantilever properly you will need to know plenty about physics like torque and rotational equilibrium.
About 1 inch just enough to assure the water goes into the gutter and not behind it.
The brackets typically hold the gutter about 1 2 1 inch away from the fascia board if u type brackets otherwise if screw through type the gutter might be almost up to the fascia board.
Each has their own reasoning for a slightly different amount of overhang.
Greater overhang becomes a point of leverage for gusts of wind to rip off your roofing.
Shingle manufacturers say to use a 1 4 to 3 8 inch shingle overhang over the eave where your roof ends.
A typical metal roof overhang may be 2 to 4 inches or even less.