Looks like you beat me to exactly what I was just thinking.
Anyway, I'd say that Human form Helbram had to have been in the 2000's seeing as how he was a rank above Gilthunder. Thinking about it now though, unlike his fight with King, he probably didn't take all of the power of the HKs, so it most likely wouldn't be the full 300. So let's just say, each Knight gave him...a third of their full power, it'd be, at the least, 2000 plus his own and, eventually Hendricksen's. That makes things a bit more reasonable to me when considering the whole Diane thing.
Quick note, I think a point we also need to consider is at which point power gaps starts to become apparent. How much exactly does 100 points add to someone's power. Is is barely noticeable? Or is it quite apparent?
Back to Diane, we, yet again, need to consider emotions and how she reacted when saw an injured Meliodas. Plus she had her Sacred Treasure which, as I'm thinking about it now, we didn't see any of the Sins with during the Eye's of Balor chapters, meaning that their power could most likely jump some more when we add them in. It could potentially end up being an issue for Helbram. These could easily double her overall power, which would be troublesome. There is also the fact that the scale of the attack was pretty large. If it were someone else, he'd probably have a better time.
Now, considering all of that, during King's fight with Helbram, Helbram revealed his true form, which increased his power according to Hauser, took all the power of the HK, which would now make that number 6000 at the least, and used the Sacred Tree, which could be considered his equivalent of a sacred treasure. His power's could now be greater than 8000, maybe even 9000. So for that moment, King's love for Diane allowed him to reach a power greater than that, and, going by the fight, it was still a quick win for King, so he had power to spare.
These numbers are very situational and have a bunch of variables that can affect them. It's pretty pointless. These numbers can rise and probably fall due to emotions, but everyone is so damn emotional these numbers can change at the drop of a hat.