Couchbase Setup Wizard Default Memory Quota Invalid When System Loaded

Right now my mac has lots of memory in use: I'm running two nodes of Couchbase with cluster_run, and have another inside a Windows image in VirtualBox. The ActivityMonitor reports

Physical Memory: 16.00GB

Memory Used: 13GB 

If I then launch an official Couchbase build (e.g., Spock Beta2), the set-up wizard creates a default value for Data Service Memory Quota which is too large, and an error message is shown:

Total quota (6844MB) exceeds the maximum allowed quota (6296MB) on node 'ns_1@127.0.0.1'.

See the attached screenshot.

When other processes are not using as much memory, the default value is correct and no error message is shown. So it seems that the code for computing default memory quota does not work correctly when the system has a lot of memory used.