When is a GPL program which runs in a web site 'conveyed'?

[pschmidt78]

Hello!
Our small company is internally using GPL software which we have
modified for highly advanced functionality in engineering
applications. So far it is clear that the code isn't distributed to
anyone outside the company, and this is why we protect the modified
sources.

We have been asked by customers to provide that special functionality
in a DIY fashion rather than offering it only through our engineering
services (delivering the results, not the tools). Because we can't
afford to reveal our code improvements, they suggested to make the
functions available from a web interface at reasonable usage pricing.
They argue that we wouldn't 'distribute' the program so that the GPL
requirements won't hold.

However, at http://www.gnu.org/licenses/gpl-faq.html#UnreleasedMods we
read

'A company is running a modified version of a GPL'ed program on a web
site. Does the GPL say they must release their modified sources?

    The GPL permits anyone to make a modified version and use it
without ever distributing it to others. What this company is doing is
a special case of that. Therefore, the company does not have to
release the modified sources.

    It is essential for people to have the freedom to make
modifications and use them privately, without ever publishing those
modifications. However, putting the program on a server machine for
the public to talk to is hardly “private” use, so it would be
legitimate to require release of the source code in that special
case. ...'

How can we distinguish when the GPL'ed program is only 'used' by the
server, or when it is publically being used on the server? If we
follow this explanation strictly, for example every web service
running MySQL (under it's non-commercial license) and allowing custom
SQL requests would have to reveal all it's source code. On the
contrary, if we take the term 'convey the program' literally, the GPL
doesn't apply, because no code is conveyed at all, and we could even
follow an approach like https://desktopondemand.com to make our whole
installation available without distributing anything (but HTML).

Both our customers and we would benefit if the outlined access to our
tools through a web service doesn't violate the GPL. The minimal
interface would consist of some input fields to define parameters and
to deliver the graphics resulting from the processed input. Where can
we find more information on how to interprete 'distributing
software' (GPLv2) or 'conveying software' (GPLv3)?

Kind regards,
Philipp Schmidt