[Aavso-photometry] Lightbox flat question

[Robert J. Modic]

I've taken lightbox flats with my 8" f/5 Newtonian for some time
now with mostly good results.  The lightbox is placed on the front end
of the tube (about 1x the scope's f.l. from the primary mirror). 
Recently, I've tried to use several short f.l. lenses (28 to 130 mm
f.l.) for wide-field CCD photometry.  When I try to use a lightbox to
take flats, the results are sometimes uneven.  Depending on the
distance of the lightbox from the lens, the resulting flats can look a
bit mottled and they don't flat field my science frames as well as I
would like.  I've experimented with distances ranging from 0.6x to 2x
the lens f.l.  The shorter distances seem to give the best results.

Is there an optimal distance to have a lightbox from a lens?

Bob Modic