It's not that important with an exact angle. It's probably just somewhere in the middle between 90 and 45 and instead of giving the exact 67.5 they say 70 as it doesn't matter with a few degrees off. It will also be dependent of the antennas tuning and the surrounding objects close by, like a metal mast or pole. To find the most optimum angle you could monitor a weak signal and then bend the radials to different angles to get the best signal. But the difference would probably be so small and hardly detactable that it would not be worth the extra effort.
The incline standards for horizontal are always 0 degree and straight up are positive 90 and straight down are negative 90. I don't know if some antenna manufacturers use their own standard.
/Ubbe