Yagi antennas require an exact set of lengths and spacings to work properly.
The lengths are all near a half wave but there is some subtle tuning to get the currents to the appropriate phase.
Spacing can be between a quarter and a half wave but different spacings affect the optimum lengths.
So your example of a 5/8 wave reflector won't work as a reflector, ...
First, the gain or efficiency of the dish will be lower than you estimate.
The outer quarter wave of the dish is substantially lost to diffraction. This is a 26% loss of area.
If it's centre fed, feed blockage will cost about 0.3 m² or another 10%, in practice double this because this is the best illuminated part of the dish.
Then uneven illumination and ...
How will you feed this dish? The feed antenna that comes on these antennas is not suitable for 70cm. The ideal feed antenna illuminates the dish and nothing else. To achieve such an illumination you'd want something like a Yagi or a horn antenna.
After building such an antenna for 70cm and attempting to mount it on the dish, I think you'll quickly realize ...