I think that this 300mb limit must be calculating using such things as
"syncdelay" mongod option, and sysctl "vm.dirty_expire_centisecs" - flushing parameters.
And also can be set using config directive.

In my case his 300mb limit probably not enough, because I use
"syncdelay=180" and "vm.dirty_expire_centisecs=12000"

See
https://groups.google.com/d/msg/mongodb-user/SZe0nCBCR80/n5aDQ2wb_6MJ