It's my preference but I think 70% is usually ok - but I haven't used it in environments with such restricted RAM capacity; For me it's always 1+ GB of ram available on the machine.
@kwladyka This is a pure speculation but the "virtual memory" thing I was referring to can be caused by "ulimit" - similar to this: https://stackoverflow.com/questions/44273737/how-to-use-ulimit-with-java-correctly The thing is that ulimit can (on linux) limit only total virtual memory address space. Maybe the google cloud machine has some default for this setting? (it's usually unlimited)
If you have ssh access to the machine you could verify that via :
prlimit --pid PID -v