The TEAC YTT-1144 and MRL 31J229 are 250nWb/m fluxivity for "0VU",... although e'thing in analog is "continuously variable", and you are free to pick your own operating levels within a certain range.
All I'm saying is that the "+6db/185" MOL fluxivity level on your (456) tape should correspond to +3VU on the meter, and the "0VU" calibration was traditionally set at +3/185 (250nWb/m) for running a +6 tape such as 456.
It's based on the philosophy that you want your MOL point to be peaking in the red on the meters, not at 0VU,... but there's apparently a lot of room for debate on that.
F/I,... the guy whom I bought the MS-16 from had his 250nWb/m cal tape adjusted to read back at -6VU, and he's running 996 tape, a +9 tape. So I said, 'You have your 0VU level set at the tape's MOL',... and he said, 'Yeah, but people love that tape compression!' [heh]
So,... my point is that some people like me want to stick to the book, with traditional philosophies, while other people tweak and use the parameters for one effect or another, depending on what they're after.
I'm comfortable with a setting that puts the the MOL of the tape you're using at +3VU, coincidentally which is also the top number on the Tascam 38's VU meter. I set the 0VU point at 250nWb/m, plain & simple,... (for 456 tape), and that's by the book. At no time do I want my MOL at 0VU. Where's the debate?
On the other hand, I've seen a lot of confusion and well intentioned misinformation about tape, operating levels and analog calibration. People are either A) misinformed,... B) on their own trip about wanting to bend the rules & do their own thing,... or C) going after a specific effect.
D) All of the above!
The beauty of it is that there's a wide range of analog calibration settings that will yield good results with a wide variety of tape. You just have to know what parameters you're working under,... and you're there.
I mean,... if you're that rabid about "tape compression", then just slam it all the way to the right in the red all the time. Don't put the tape compression threshold at 0VU, because that doesn't seem rational in my by-the-book mentality.