Variable amount of free space indicated in "Manage Apps" When I go to "Manage Apps" and view the "Downloaded" section, the indicated amount of used space first shows a high value then drops slowly while the system calculates the space used by each app.  After around one minute it finally displays a value which is up to 100MB less than the initial value.  

I had always assumed that the final value was the correct one but recent experience suggests that the first value displayed is what is actually available for installing apps.

So, can anybody tell me what is actually happening here and how I can reliably measure the free space?
You should be able to execute the df command from within any terminal app or via adb shell. It would list up all available file systems and their "free space" (df stands for "disk free"). But you would need to know what is what there. To give you an example from one of my tablets:

shell@android:/ $ df                                                         
Filesystem             Size   Used   Free   Blksize
/dev                   430M    64K   430M   4096
/mnt/asec              430M     0K   430M   4096
/mnt/obb               430M     0K   430M   4096
/system                674M   331M   343M   4096
/factory               127M    18M   109M   4096
/data                    2G   505M     1G   4096
/cache                 127M    18M   109M   4096
/storage/sdcard0         4G   650M     3G   4096
/storage/sdcard0/external_sdcard    29G     2G    26G   32768


So just like Cinderella you have to pick out the good ones, but simply can ignore the rest (or you can take a look at Android Folder Hiearchy to learn the important ones):


/system is where your OS and its system apps reside. Rarely interesting to the end user, as it's not writable (except you can make it such on rooted devices)
/data is your "internal storage"
*/sdcard* (differs between devices and Android versions) your internal or external SDCard (see next item)
*external* (may be external_sd, external_sdcard, ext_sd, ...) is your external SDCard. If that entry exists, the one mentioned with previous item for sure is the internal one.


So in my example there's enough free space available: 1 GB on /data (internal storage), 3 GB on internal SD, and 26 GB on the external. As these values are yielded by the system itself, one should be able to trust them :)

Remark: I ommitted some of the df output to make it better readable.