First off, I'm not sure what SDI's version of SAC would be useful for ... a baseline is only any good if it applies to something. Taking measurements while sitting perfectly still on the surface have no real applicability to what goes on while you are diving.
My definitions of SAC and RMV are provided in my
gas management article ... which is how I teach them. Essentially, they are two representations of the same value ... expressed in different ways ... and they do not represent a baseline, but rather a range of values that will be dependent on the conditions in which they are being measured.
When we breathe on the surface, we don't have a baseline ... our breathing rate will be dependent on what we are doing ... sitting on the sofa reading a book, walking, running, sleeping ... those will all give you different consumption rates. The same applies underwater. The only thing that truly matters is having some understanding of what the high and low end of your consumption range are, and where various activities will put you on that range.
SAC and RMV both represent that range. The difference is that SAC is expressed in terms of pressure per minute, and is dependent on the cylinder you are using ... while RMV is expressed in terms of volume per minute, and is independent of the cylinder.
RMV is used for dive planning ... because we know the volume of our cylinder.
SAC is used for dive execution ... because we know how much gas we have at any point in the dive by looking at our pressure gauge.
We don't develop an understanding of how different conditions affect our consumption rate by establishing a baseline ... we do so by tracking our consumption over the course of multiple dives, in different conditions, and considering why each is higher or lower than the others. In other words, you look for trends.
Gas consumption ... like so many other aspects of diving ... is not an exact science. There are a lot of different factors that will affect your gas consumption on any given day and any given dive. Understanding what those factors are, and how they affect you, will give you a much more accurate "picture" of how to plan your gas needs than sitting in your living room breathing off a scuba cylinder ...
... Bob (Grateful Diver)