Inside corner under valley. I put a six inch gutter and downspout on. I did not put a splash guard on because in the fall everything fills with leaves so it is pointless. I just put down mulch and to see the water over shooting the gutter and wash my mulch away was sad. Maybe I need to reconsider the splash guard idea, at least until fall. It is too much surface area going to a gutter that is too small length wise. I have never liked the idea of doing anything to the metal valley itself. Such as putting little pieces of flashing to slow down the flow. Now I am open too suggestions. I was wondering if I put a 4 inch why at the bottom of the downspout if that would act as a vent to allow more water too go down faster. I have even thought of laying chain in the valley to slow the water. Is this just one of those no win situations?
Splash guard is the best solution if its just blowing past the gutter.
Length of gutter vs roof area doesn't matter, as long as the drains are adequate. For a lot of sf into a single gutter use larger downpipes, 3x4 if your area uses rectangular, or 3.5x3.5 if square.