Tricky!
I have always suggested very low IBUs for low-alcohol beers, because it is so easy to overdo it. Suggest using the BU/GU (GU = OGx1000-1000) ratio and a bit less ('cos they finish with high FG). So the Centennial Blond, which is fairly lightly hopped anyway (22-23IBU, it's a "pseudo lager" or "blonde ale" with OG 1.040) has a ratio about 0.57. A low alcohol version might have an IBU of only 8.3, a ratio of 0.41 (OG 1.020). That should be about right (when I get around to brewing it!). I get the low IBUs by cutting the hops to 2/3rd and shifting the timeslot back (so the 8g at 20 boil minutes remaining, ends up as [EDIT: 5.5g,] 20 minutes whirlpool/steep hops at 85C).
Ermm ... you didn't expect a straight answer from me, did you?
Then you get Big Drop quoting (or once where) quite outrageously high IBUs. And some of their beers are very good if very bitter, and some taste of wet corrugated cardboard boxes 'cos the hopping is too high. So, I haven't got the answer yet (but aiming low is safest!).