I have clay soil. Always preferred to lay soaker hoses on the ground and covered over by mulch. This keeps the mulch dry, the ground moist, and the hose protected from the sun.

Don't have any specs on soaker hoses, but maybe some of the info for dripper line would help. Netafim makes poly tubing with built in drippers at various intervals (see link). It looks like sandy soil requires a higher GPM with a closer spacing due to faster downward water movement. Seems like the same would be true for soaker hoses, except that instead of regularly spaced drippers, the whole hose drips. So try 12' spacing and regulate the depth and spread of water infiltration by adjusting the flow through the valve/shutoff and the length of time that you water.

Here is a link that might be useful: Techline CV Dripperline