Most of the ones that I have seen have been a nice formula eg...

Most of the ones that I have seen have been a nice formula eg you get the first $5000 applied for and then 20% of the rest up to a limit. SPP are generally a blunt instrument. However, recently another of my companies used a lot of discretion and based it on the amount of stock held, the length of time held, whether shareholders had sold stock etc. To be honest, it peeved me because if they want to base it on those things, they should do a rights issue which is proportional whereas SPP are designed to treat everyone the same regardless of whether you hold $500 worth of shares or $1m. The SPP plan prospectus should give a bit of an explanation of how they will scale it back.

Bottom line though is that it is up to the company how they choose to do the scale back but some details should be in the prospectus.