Ratio between bullet speed and spawn time

What is the "golden ratio" between the time of spawning enemies (every x seconds) and the bullet speed for a space shooter?

I would like to start with a baseline of bullet speed = 50

then i would like to increment it by 25 every 10 points (i got this down)

What i do not know is what should be the best time (in seconds) to spawn enemies for a speed of 50, and how should that change while the bullet speed gets incremented?

all this for a vertical space shooter.