As noted, chipping is a given if the crete isn't flush with ground. It is mitigatable tho.
Generally, the steeper the better, mostly cause you want the concrete to be as thick as you can, for as close to the asphalt as you can. Long, mellow feathered seams are going to chip faster than a steep, thick corner/seam that is sharp and basically squared off.
For the angle, the above idea of having your nose touch is smart. If you do end up feathering, don't use concrete. The rocks will fuck ya. Either sift the dry cement out or buy portland and basically make a patch where the asphalt meets the mud.
Make sure you cure the fuck out of it. Once the crete is hard enough to not wash away, keep it wet over the next couple days. I'd give it at least a week before skating
There might be better advice, but just my thoughts after doing this same thing at my asphalt lot DIY.