I've gotten around 1429. i can't log in to check at the moment.
Between level 1 and 20 you gain 307BP
You start with around 600BP and all +/- differences are maintained level to level save for HP which grows, but not by much
Primm Weapon ungrinded counts for 150BP. Cattleya is around 200BP ungrinded
Primm Armor ungrinded is 5 while Cattleya is 10BP ungrinded
Each grind of weapon is 2BP and each grind of armor is 1BP... more or less. Sometime it only goes up by 1 and sometimes goes up 3, but it all evens out to 2BP/grind
It looks like that there is no statistical calculation to come up with BP for example 2PP doesn't equal 1BP. At least it doesn't look like it as you can see that Primm adds 10HP and 8 Defense and gives you 5BP while C/Stamina gives you 5HP/3BP while Alts Soul gives 5HP/5BP but Ael Note A gives 5HP/3BP. This establishes at least 3 different BP costs for HP so the conclusion is that each Augment gives its own set amount of BP. I've checked this against number of Augments in each item as well and the Augment itself gives only x amount of BP, not the amount of augments in the armor.
There is probably some internal calculation but I think the numbers are hardcoded. That some cost less and some cost more indicates to me that there is probably something about those particular augments that makes them special or more valuable rather than they're just sorta making them high BP for the heck of it.
So how to get BP
Leveling if it remains consistent is roughly 15BP/level
Learning skills is is 3BP/skill (not sure what happens with subclass skills, haven't tested it yet but considering the 15BP above seems too high it probably does have 2 or 3BP per subclass skill)
Weapon is 150 plus roughly 12-15 per rarity increase. Grind to 40 gives you +80BP
Armor gives you 5 plus roughly 1 per rarity increase. Grind to 40 gives to +40BP
Augments add 2 to 10BP depending on the Augment (it looks like I haven't checked the higher ones)
This would mean a lvl 100 should be 1800 at base
I assume 8 regions by level 100 with 20SP each so 480BP
15* Weapon should be around 300BP ungrinded and 380BP grinded to max
15* Armor should be around 20BP ungrinded and 60BP grinded to max
Augments will likely get no higher than an average of 5BP per rarity so a 15* Augment will be like 75BP, but probably not.
Base + skills + weapon = 2660
This would indicate that Armor probably one of the top sources of BP... assuming that the numbers are roughly accurate cuz Armors all together would be like 1380BP and that's about a 3rd of your max level...
But for now, what adds the most BP is leveling from 1 to 20 (adds 300) and Grinding your equipment which adds 200BP. Augments and increasing weapon rarity barely does anything relatively from my testing out and checking of augments and such. I don't have anything that magically increases BP or percentages for set bonuses yet so can't say anything about that. And then I also don't know what unlocking the potentials does cuz I forgot to check that all the times when i did it.
Besides just basic BP calculation the take away I have from this is that you should be careful to realize that someone could have equivalent equipment but higher BP due to some augments having the same stat increase but different BP increase.