Just going by the maths, with the PB1 and guns alone the F8 should be able to get about 12 buildings (12.3%). Possibly more, as I don't know how much you can increase that by the PB1 splash damage radius, which I haven't tested. And then you would have the 500kg bomb, too.
20% should be definitely doable.
However, I have some kind of feeling that we didn't hear OP's full story yet...
^
ADDENDUM:
A quick and somewhat sloppy test achieved 37% of damage against a radar factory.
With better bomb placement you definitely should be able to push past 40%