I'll try and help but don't have real
experience on this topic, my knowledge has come from reading a bit.
First knowing it's steep land for sure I would leave a larger forest strip to help the soil be protected from erosion.
Second the width of the spacing between the forest strips is directly realted to tree height, and sun orientation. I would calculate the shadow the
trees cast and see that the shadow of the last trees from the first row touch the base of the trees on the second, but you have to think always of the erosion posed before.
oreintation east/west if it is possible can help somewhat with the spacing being you gain more light in the alleys and can have a good yield from what you seed in the alleys.
I tried hope it helps you a bit. search on the web something about alleycropping in agroforestry or similar some ideas may come up