Java Service Wrapper

It would be very helpful, if we could define that the wrapper sets the -Xmx
flag in some relation to the available physical ram on the system.
Our Application runs with 32MB for small business, but may require several
GB's for enterprise systems.
Since -Xmx must always be smaller that the available physical ram, we had'
to know in advance, which (absolute) maximum value is possible. We don't
want our customer to edit that setting. The Application should just use the
available memory....
I know this is a JDK-issue, but people at Sun did not fix this for 6 years
now. I think there is no hope.

I'd like some setting like:
java.system.maxmem=90%

On a small server with 512MB Ram, this would end in the JVM-setting
"-Xmx460m". A LArge Server with 8GB Ram would then become a "-Xmx7372m"
setting.