The Sagulator calculates the amount of sag (deflection) based on the type of wood and dimensions.
Use redundancy wherever possible to prevent things from failing. A structural element should be glued and screwed in. If one fails the other should have a chance at taking over.
Use structural fasteners to fasten load bearing elements that need to be able to
Mind the distance between holes and the edge of the wood. Rule of thumb is the distance between the hole and the edge should be 2.5 x D (D is the diameter of the hole).