SNMP-Test for Free Space or Usable Space

Hello,

I want to monitor the free space and/or estimated usable space of our SC3020 but it seems to be impossible. The only information I found in the MIBs were these:

In the GUI I find those helpful values:

So, is there any chance to get this values via SNMP? If not I have to try to calculate the values, perhaps someone can clarify that my calculation is correct?

Free Space = scDiskFolderSUTotalSpace2 - scDiskFolderSUSpareSpace - scDiskFolderSUUsedSpace

Thanks!