I'd suggest doing it partially by comfort level (will folks be overwhelming by the number of options?) and monsters you want to throw at people
I think that campaign, from what I know, would support epic 4e play just fine and you could safely end it at any point 20 onwards... I'd personally be tempted to at least get to 24th for the final battle so you can have some 'When I die...' type action going on.
Ignoring *1.5 math, 11th and 12th are nice levels in terms of getting a fair amount of toys and having things to look forward to next level. In converting the shackled city for 4e that I'm doing, I looked at what monsters I wanted to use and how I expected to shake out levels, then picked a level I thought would work for where they were (11th, in my case)
I easily could have gone lower if I'd wanted to end the campaign at a lower level (which I considered) or give out less levels per adventure (I like giving out levels pretty quick though)