Just take the slate out of the tank after you notice the male has been fanning them for a bit. Then you can reasonably be assured he fertilized them. Put the slate in a very clean container with an airstone, and add Methylene Blue. The MB is a fungicide and will keep any bad or unfertilized eggs from fouling the whole spawn. I noticed you use long slates. Years ago I used tall glass canisters made for storing spaghetti. The glass was much easier to sterilize.
Angels are hit-and-miss as parents and some pairs are beasts as guardians, some could care less, and some just need to work at it. The ones that do get it right shouldn't be given much more than 3-4 weeks with the fry as they'll most likely get the itch again, and will either lose focus, or will eat the fry.
Careful not to add too much water movement especially once free-swimmers. Once the yolk sack is absorbed, just move them on to progressively bigger quarters. At about pea size comes the tough job of culling.