I don't quite understand your calculations... But yes, Bell should be level 6 in terms of physical prowess. However, he would lose to other Level 5s due to lack of experience.

level 5

①800 x ②4(lv) = 3200 + ③2000 = ④5200 (⑤high + 800=6000)

①adventurer status 0-999

② level to be considered

③ bonus when leveling up

④ low adventurer all stats with 0(zero)

⑤high experienced adventurer

I forgot to describe my calculations. But I just added up each ability. Strength, Dexterity, etc. and compared it with adventurers who have status with 800, 950 and 999. The last one I refer to Ottar.

Why does level 4 have a level up bonus of 2000?

If we considerider the War Game of MS 8 & 18 as Mirror of each other & look at the similarity existing between Bell vs Ottar & Bell vs Hyakinthos. Bell after the last status update he receive from Hestia must have the stat equivalent of a low level 6 (except for agility & magic, the former being higher & the later being lower)

I also made some calculation based on Season 3 fight, Bell needed to have most of his stat at D rank at Level 3 to be the raw equivalent of a level downed Dix, considering the final status Bell has at level 4 was far above the previous by a range (except for agility stat who saddly has decreased by 35 point) between 242-375 points for each statistic so to redo the same above my current level raw ability that Bell do against Dix, he would need most of his stats to be at F or G which was the case after Hestia update during the War Game.

Of course, this is just an estimate that didn't take into account Bell's final stats at level 3, disregard his magic stats & mostly keep it simple but as a default margin this is quite sufficient.

If Bell were a level 6 he wouldn't lose to any level 5 unless said level 5 had some super ability that would level him up; experience is only decisive when it comes to adventurers of the same level, but when there is a difference in level, status is decisive. Such was the case of Tione against Asterius, as basic as her fighting style was compared to an expert like Tione, he would have beaten her since her blows barely bothered Asterius.

In all the comparisons between Bell and other level 6s (Tiona, Tione, Hedin, Ryuu and Mia) Omori has said that Bell is inferior. It may be because Bell doesn't have the combat skills of these geniuses despite having the same level, but it is also possible that he is simply a level 5 compared to level 6s, hence his inferiority. My point is that there needs to be some explicit demonstration by Bell that proves he is indeed level 6 instead of just assuming it.

But either way, it's a waste of time trying to figure out through numerical calculations how powerful Bell should be; the boost due to his stats is determined more by Omori's circumstantial will than any numerical calculations, which is why in some battles he has shown a gigantic boost and in others a tiny one.

The issue is that so far Bell hasn't fight any level 5 & the level 6 he got weren't exactly the average type (Tiona defeated her teacher & Tione maybe would have done the same while Ryuu has now so many boost she can spair with that anomaly of an elf called Hogni).

You're asking for something pretty unfair, it's not like we can pull out of nowhere a level 6 who has no skills or magic that could increase his fighting ability while being only average in terms of fighting technique to battle agaisnt Bell just to see if they're on equal footing, the best possible option so far would be fighting a Level Boosted Aki.

Yeah yeah Omori is playing favoritisme & isn't even subtil about it but that's not an excuse to just give up. It's not a question about what Adventuer Bell would surpasse (that answer has been given a long time ago) but more what kind of antagonist or monsters he will face in the Dungeon could be expected as challenge (In MS 12 we got a Moss Huge who ended up reaching Level 5 power).

Bell's update before continuing his battle with Ottal put his agility in the level 6 range and of course he received his post-wargame update which could be a few hundred points more than what we are aware of. Bell's agility stat is important because it is what his fighting style revolves around, and the other stats compliment it.

As for Bells experience in combat, I think that was one of the primary focusing points of volume 17, which was to train Bell how to fight properly, sure Aiz had been teaching him before that but the Freya Familia has far greater experience then she does, plus Bell's teacher is a former king (Hedin Selland) and there was even an instance of him teaching his familia members how to fight better, so I really don't see the point of calling his experience into question anymore.