Homebrew PCBs

In cases like this, you really should consider buying a transmission
step wedge.  I got a stouffer 21-step wedge for $7 plus s&h and it
REALLY tells you how well (and accurately) your process is.  For
example, my inkjet gives me an 8-step range, but my laser printer only
gives a 3-step range.  Even the worst inkjet film I can print is going
to be better than the laser printer.

I found that using "standard" software and films won't give you truly
opaque prints.  I ended up writing my own printer driver just for films,
and buying Jetstar films (perhaps anything with "ultra fast drying" in
the name should work, but YMMV).  The specially coated films let me put
much more ink than standard software would, resulting in a much darker
film.