Here, laterals look to be 20' to 25' apart, tieing into a mainline in farm fields, but they don't use gravel. Gravel definitely helps. The closer to the surface, the quicker it will drain. But, also be aware in the summer when it gets dry, you may have streaks of dormant grass, from being too dry, if gravel is too close to the surface.
Personally, I put fabric over the gravel, to keep dirt from sifting down through the gravel. I get the premium landsacpe fabric, cut it to double the width of the trench, and fold, so there is a double layer of protection. I cut it with a large utility knife I got at Lowe's. I tried cutting it with radial,and table saw's, and it just melts the ends, and makes for a PIA to unroll.
Some insist on using regular geotech fabric for that job, but years ago, they just used straw. Dig it up 25 years later, and there isstill straw there. If the air can't get to it, thefabric will work fine. Last Fall I tied into an existing tile I put in 10 years ago. The fabric I used then, still looked like new.
