Augmented fourths and diminished fifths

There's been a lot of debate about the size of these intervals lately.
I'll ignore everything that has gone before, and state my own position.
It can be summarized thus:

1) If context is everything, spelling is nothing.

2) The characteristic size of a diminished fifth is 36/25

3) If you want 7-limit tuning, you should say so.

By (1) I mean that, depending on context, a diminished fifth can be
anything from 583 and 612 cents, the former being for a natural seventh
chord, the latter for Pythagorean intonation. That means the uncertainty
in diminished fifths is greater than the difference between diminished
fifths and augmented fourths. So, if you expect the size to be determined
by its functional context, the spelling is merely to underline the
function. Either you decide what size the intervals are going to be, and
spell accordingly, or you spell according to the function, and let the
tuning take care of itself. You can't have it both ways.

(2) is simply demonstrated. A diminished fifth is two minor thirds. The
characteristic tuning of a minor third is 6/5. So, the characteristic
size of a diminished fifth must be (6/5)^2=36/25, or 631 cents. So, a
diminished fifth is sharper than an augmented fourth. This makes sense,
as fifths in general are sharper than fourths in general.

Say you have a chord progression involving I, V, ii and V7. For I, V and
ii to be tuned to 5-limit intervals, and only 7 notes to be used (the
simplest case) the 7 in V7 must be a ratio of 9/5. If you want the 7 to
be 7/4, 49 cents flatter, put a \ on the score. Hence (3). If the \
isn't there, the performers can tune it however they want, which may not
be what *you* want. So tell them. This will leave the tritone as a
sub-diminished fifth, equivalent to an augmented fourth in meantone.

Such "quartertone" directions make it easier for the composer as well as
the performers. If you find yourself writing a progression where Bb\
transforms into a plain Bb, you'll know there might be a nasty comma shift
when it comes to be performed. That way, even if the singers ignore the
\, they'll find it easy to get right. It should also remind you not to
give the Bb to any piano or guitar accompaniment.

If you expect JI in performance, you could even try writing in the
syntonic commas, with a schismic or 72= notation. But even accurate
meantone is a start.