So what? I do not know about HP5+ but it is not unusual to get a Dmin of
about 0.3 for 35mm films because the base has a grey dye in it to prevent
light piping. Fb+F, while it must be known, is not of any significance
unless it is high compared with product specifications, and indicative of
improper storage.

What are you talking about?

Ansel Adams used to recommend a NET density of about 0.1 above Fb+F for
film exposed to Zone I. (I do not find this good for my work, and expose
so that NET density of film exposed to Zone V is in the range of 0.85 to
0.9, which results in a NET density in the range of 0.25 to 0.3 (for TMax
type films).

Why? What difference could it possibly make?

What dilution time and temperature are you currently using.

Is the film sheet film or roll film?

  I think you are confusing two things. Log 0.1 above base
+ fog is the standard speed point for the ISO speed
measuring system, the actual B+F level is of no
significance. Most 35mm negative films have a pigment in the
base for anti-halation and anti-light-piping. This pigment
has a rather high density, typically around Log 0.3. This
has no significance other than it about doubles the
necessary exposure time when printing.
  Whether log 0.1 is a good density to place minimum shadow
density depends somewhat on the shape of the film's toe. The
idea is to get the shadows which are to have some detail up
on the toe far enough so that the contrast is adequate. Loyd
A. Jones, of Kodak Research Laboratories, determined
experimentally that this should be where the toe contrast is
1/3rd the straight line contrast. Less exposure results in a
loss of shadow detail, greater exposure makes little
difference in tonal rendition.
 As far as fog goes: if film is old and known to be foggy
the exposure should be increased. When this is done the
overall density is increased and the lack of shadow detail
from the fog is compensated for. Such negatives print fine
but may require longer printing exposures.
 Anti-foggants will reduce the fog density but also tend to
destroy some of the latent image effectively loweing the
film speed, exactly what the fog does. So, the only
advantage may be somewhat shorter printing times.
 Don't worry too much about where the minimum exposure is
in terms of densities. The main thing is for it to be far
enough up the film curve to yield good detail. In Jones'
research he found that tonal rendition remained the same
over a rather long range of exposures once the minimum for
good shadow detail had been reached.
  Minimum exposure is still important, especially for 35mm
film, because the grain tends to increase with density and
sharpness decrease. So, the ideal is to find the minimum
exposure which will print with good shadow detail. For
larger formats, where absolute minimum grain and absolutely
maximum resolution are not so important, its usually safe to
overexpose a bit to insure good tonal rendition.
  Developers are much the same in fog level. The is some
reduction of fog in developers with a lot of restrainer in
them. Used developer generally has less fog than fresh
developer because of the accumulated bromide and iodide, but
the effect on the latent image of the restrainers also
lowers the effective film speed.
  Rodinal is actually a fairly low fog developer but it
also tends to reduce effective film speed a little, not
much, perhaps half a stop.

Rodinal on HP5? WHY?

In any event, Rodinal is a low-speed developer, not at all suited for
fast films. You lose about 2/3 stop in shadow speed. You must be using
too high a concentration of this stuff, or too high a temperature, or
excessive time.

 I think that possible with a less active developer
although .16 fb+f with that sheet film I'd think quite good.
If you compound any of your own developers try D-23, Beutler
or, FX-1. The last two are similar to Rodinal and like it
can have additions of sulfite made to suit one's taste.  Dan

  Of course I mean log 0.15 and log 0.16  What's a dot
between friends.