Finding the best distance from the dish to couple the most signal for the Bi-Quad is a missing factor.
Parabolic dishes (lets just talk about prime-focus for a moment, because they're more intuitive) are not rocket science.
Parabolic dishes have a given FD ratio, which determines exactly where the perfect focus is for a given diameter dish.
You can move in and out from there along the focus axis to maximise the effectiveness of the dish to suit your feed.
Ideally, you want a feed that produces a perfect conical radiation. Reality is that most antennas have different radiation in the H and E plane, so it'll be a compromise.
If your antenna/feed has a radiation angle of (lets say) 30 degrees. If your feed is too close to the dish, you are under-illuminating it and only a part of your parabolic surface is contributing.
If your feed is too far away, you are over-illuminating the dish, and some of your power is going straight past the dish and being wasted..
Perfect illumination sees all the dish being used. The ideal focus can be calculated, but it's still a compromise and fairly easily found by experimentation, measuring the signal strength. It's arguably easier to use the dish in a receive mode with a source quite some distance away (preferably hundreds of wavelengths away), fix the dish and adjust the feed distance for optimum signal strength.