Okay, so the first thing I'd like to clear up is how Buggy is scaling to the calcs, regardless of which calc we use.
Durability: At least
City Block level+ (Is fine after being hit by the
Buggy Bomb Explosion twice. Took multiple blows from Monkey D. Luffy)
1) The calc that is being used currently in Buggy's justification is
this one which doesn't calculate the actual explosion of the Buggy Bomb, but rather the kinetic energy of the cannonball plowing through multiple houses and destroying them (prior to the explosion) and getting a City Block level+ result.
2) The actual explosion of the Buggy Bomb at the time of the Orange Town arc is
this calc which is Large Building level+.
3) Of the feats where Buggy is endures the Buggy Bomb's explosions, I can't find any direct evidence that Buggy was hit head-on by the cannonball and therefore tanked the kinetic energy of the bomb prior to the explosion.
3.1)
Here is the first sequence from Chapter 11. Buggy is evidently within the explosion radius as he is scuffed up by the explosion and coughing from the smoke created but there's no sign of it actually hitting him directly.
3.2)
Here is the second sequence from Chapter 15. This one is even worse as a durability feat because not only is it not shown that Buggy was hit by the cannonball, but he also used some of his own men as human shields so he's not directly harmed by the explosion.
So of the two feats of Buggy "being fine after being hit by the Buggy Bomb Explosion twice", the first one should be using the explosion calc instead of the KE calc, and the second one isn't a valid durability feat.
So my proposal is that we downgrade that version of Buggy to
Large Building level+ from
City Block level+.
The second thing to get to is which calc do we actually use for the Buggy Bomb explosion, and for that I think that using the actual calc from the same arc where Buggy performed his feat is far better than backscaling from some 200+ chapters later. For the later feat, we don't even know for certain that it is a Buggy Bomb that he used given that it isn't named as such in the manga as far as I'm aware, and he might have improved his explosives for all we know even if it was in fact a Buggy Bomb. It was 200+ chapters after the Orange Town arc after all.
Unless there's some convincing reason for why we should backscale this calc all the way back to the Orange Town arc, I think that my proposal would be best and we just downgrade Buggy slightly instead.