Herc had B+, A+ with Mad Enhancement, but his base parameter was B+. Modifiers are temporary, ranks are permanent. His rank is B with a modifier. 

The "Saber can Prana Burst from B to A" is fanon. Prana Burst doesn't make her parameters better, it's what gives her the parameters she has. Without it, her STR and whatnot are all "1". I'll get the VN quote when I'm back at my dorm if you want, on my phone now. 

But yeah, it takes a girl with a dragon powering her and Rin-level Master support to hit A rank STR. Herc, who is Herc, with the best possible Master and a Western Europe summon, gets a rank of B. 

Base A rank STR is hard to get. You can get it through boosts like Natural Body or Mad Enhancement, but at base? Really hard.