Kathleen,
The amount of pressure that is enough is the amount neccessary to overcome the
resistance to stretch of the circular muscle fibers around the nipple.  This
varies for individual women, those who leak spontaneously will often be able
to obtain milk with those lousy little handheld "electric" pumps, and moms who
never leak are likely to have difficulty with the lactina, and sometimes with
the full weight pumps as well.  These moms are better off with compression
rather than suction, so hand expression may be better for them.
        As for "is more better" suction is not dangerous to the intact nipple
if it is applied for a very short period of time.  Egnell, the inventor of
both the Ameda/Egnell pumps and the Medela classic, made a study of this issue
and designed his pumps within safe parameters.  This is why moms can get such
damage from handheld pumps:  they often need to evacuate air several times per
cycle, exposing the nipple to up to 5 seconds of unreleived negative pressure;
and some have no mechanism to prevent unsafe pressure levels, if the mother
does not correctly release the pressure periodically, she can get into
trouble.
        Human babies use mainly compression to extract milk from the breast,
they use negative pressure mainly to keep the teat (nipple and areola) in
their mouths.
Hope this clarifies the pressure issue.
Catherine Watson Genna, IBCLC