With flowers used as a late addition you can rescue them then use them in the next brew as part or even all of the bittering addition to make use of the goodness still left in them. I believe that a lot of old UK breweries who use a hopback do (or used to do) that.
I used NZ Hallertau Aroma Flowers myself in a similar way a few years ago, but did it arse about:
I used boiling water and a French Press to extract flavour and aroma, as in a hop back. Ended up with a litre of green liquid that I put in the fridge.
Then I used the "spent" hops in the Boil and despite their steeping they still turned out nicely bittering.
I'll rephrase that: very adequately bittering for the style. On pitching in the FV I chucked in the jar of flavour/aroma extract and it turned out a very nice blonde ale in the end.
Problem is that whilst the method works with flowers, it's real "seat of the pants" stuff and no way of really calculating what IBU you are going to end up with.