My usual procedure is to dig down along the trunk, find the first set of lateral roots, and cut the (substantial) taproot just below those. That gives me a few feeder roots, but not a large amount. Those few feeders are more than enough to let the tree recover and thrive within the first year post collection.
An exception: coast live oak usually develop a large bulge in the trunk just below the soil line. If feeder roots are coming out of the bulge, or at the bottom of the bulge then no problem. Sometimes the roots are substantially below the bulge. If you cut below the lateral roots anyway, you end up with a big bulge in the middle of your trunk and have to essentially collect the same tree again in a year (or ground layer). You can cut at the bulge with no roots and get the tree to survive but it will take longer, at least two seasons (?), to be able to work them. So if the lateral roots are substantially below the bulge, your choice is to collect what is essentially a large cutting, or move on to another tree.