What is the exact ratio one would use to convert Miner speed from
Hashes per second to
Shares per second, assuming Difficulty 1 shares?
The approximate ratio which is usually used in calculations is 2^32 hashes per share. If you want the exact ratio, it's 2^48/(2^16-1).
"The average time to find a block can be approximated by calculating: time = difficulty * 2**32 / hashrate"
This applies also to shares, setting 1 as the difficulty.
At 1 MH/s, you can expect to find a share every 2^32 / 1000000 = ~4295s.
Inversely, you will find (1000000/2^32) = ~0.000233 shares per second per MH/s.