Hi Paul,
This is fairly common in new music. I've found the most consistent way to get it to look good is by using overrides. Here's how I code it:
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\override Stem.stemlet-length = #0.5
\override Stem.details.beamed-lengths = #'(4.75)
f16->[ f r f] f-> f f f
\revert Stem.stemlet-length
\revert Stem.details.beamed-lengths
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You can adjust the beamed length to whatever value you prefer to keep the rest centered on the stave vertically. You don't need to revert the overrides until you're done with the stemlets.
It's a bit tedious to do this for every beamed group, but it gives you fine control over exactly how they look.
Cheers,
A