You could try using 5 steps rather than 100 then a phrase like 'at least' or 'minumum available'.
By only changing your graph when one of the main chunks is completely full the user will only be able to deduce that there is somewhere between (in the example) 2 and 3 chunks of space available.
Your graph for either 41% of 59% would look like this. They would know they had at least 40% but not as much as 60%:
download bmml source – Wireframes created with Balsamiq Mockups
Where as one for 61% would look like this:
download bmml source
By breaking the graph down to the level of granularity that the system is able to report and no further the user would be given exactly the amount of information available. They couldn't be misled as you wouldn't be assuming and displaying a value as you are strongly implying with your example.
You could also number them 1-5 and add 'currently filling chunk n'