TKM, my friend, to parrot the others, I think you're over thinking this. The example refers to a 40-yard shot crossing at 90-degrees. When do you encounter a shot like that in trap or Skeet? If it's a straight away trap shot, there's no horizontal lead. If you're on the Skeet field, the range is closer to 20-yards.
There are some short cuts to learning the leads, but they still must be learned. But, consider this: If a target takes about a 3-foot lead at 20-yards and a similar target at 40-yards takes about 6-feet, then the angular lead you see at 20-yard will be about the same angular lead as you see for the 40-yard shot.
The trick is learning the lead (as an angle ahead of the target) for the various presentations. Take Skeet as an example: Shooters talk about learning the leads for all the station. In reality, the physical lead is the same at each station (except #8) -- the target takes the same time to get to the center stake at each station, and each station is the same distance from the stake. It's the angle (to get that lead) that's different from each station. Once you learn the general lead angle for various presentations, then you've got to get focused on choke and load selection based on the target's range.
Skeet and trap are simpler than clays because the tried and true loads and chokes have been known for some time (of course, there's always someone trying to re-invent the wheel). With clays you have to know the leads as well and the optimum coke and load based on a changeable target distance. That's why it may be recommended that newbies learn the lead angles at Skeet and trap to make learning clays a little easier.