Determining actual free space on partitioned volume

I’ve set up my Drobo following the instructions for setting up a dedicated Time Machine partition. I currently have two 1TB drives and have configured a 4TB volume which is partitioned into a 750GB partition for Time Machine and the remainder as a separate partition. Is there a way that I can easily see the remaining actual space on the non Time Machine partition?

I can calculate this by subtracting sum of the Time Machine partition and the used diskspace on the remainder partition from the total space available to the Drobo but it seems a little cumbersome. Anyone know of a neat way to get this information?