The Spit1 and Hurri I are the best tools for deflection shooting training that I have found.
It's sure helped with high deflection snapshots when I've flown cannon armed planes.
You can set the convergence to about 350-450 for the .303s and then "hose" in an arc through the area you expect your target to cover, starting from under the target aircraft and up through the target area.
The rate of fire and number of rounds in the air really helps when you do connect because the target lights up like a christmas tree with a faulty inverter

It gives you a starting point to practice with the different trajectories of the different weapons, and translates as a methodology to the .50s really well.
The best I've done is 3 kills landed in one flight with a Hurri I (no reloads) and with practice I plan to get better. Getting in real close is the key.
Word to the Wise: Don't HO with .303s

palef