How far should roof overlap gutter?

I am going to hang a rain gutter on a pole barn with a steel roof. I have a K-style aluminum gutter. The steel hangs quite a distance beyond the wall, so I got the hangers that strap to the top of the roof. There is no facia board along the roof edge either.
My question is this. When I use those strap on top of the roof brackets, I can move them forward or backward. Either the edge of the roof will drop into the center of the gutter, or I can push the brackets forward so the edge of the roof is closer to the back of the gutter. I'm thinking it should be pushed forward or during heavy rains, the water will jump right over the gutter. Is there some sort of rule of thumb measurement for this?
PS. Yes, I am aware that using aluminum gutters on a steel roof is not the best idea, but I got the gutters for free from a building that was being torn down, so I'll use it on this barn. My plan is to put a strip or innertube rubber between the painted steel roof and the aluminum strap. My screws will also be neo-screws with neoprene washers, so there will be little chance of corrosion between them.
Thanks
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On Jun 11, 7:17 am, snipped-for-privacy@invalid.com wrote:

3/4" to 1"
R
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