It's not complicated. Whatever you set in the FMC cruise page, you also set in the FLT ALT.
For an intermediate level off, do you change the FMC cruise page? No. So you don't change the FLT ALT.
For a step climb or descent, you DO change the FMC cruise page. So you also change the FLT ALT.
Where is the manual does it say to set the highest expected cruise altitude?