I've done this before and it works more or less for a few steps. However
the real problem if you haven't go a densitometer is in deciding what is
density 0.15.
When you get a lot piled up I think reflections on each surface start to
cause a problem, as does dust, scratches etc.
You would also need to develop so that your straight line has the correct
gradient.
Its actually easier to do a test strip or strips at various times,
develop, measure the densities, draw a curve of density v time and read
off the times you need for the 0.15 steps under the same conditions.
As I said before if you want a reasonably accurate wedge pay the 8
dollars!
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/