In the software load on our FMC, the difference between OPT and MAX is usually around 700ft.
Generally, when planning a step climb, we normally enter a level approx 300ft higher than the planned step level (ie, if we want to step up to FL380, we enter 383 as the desired step FL). It means you wait a little longer for the step climb point, but it also provides a slightly better margin than climbing immediately to MAX level.