Why does Time Machine estimate a 109GB backup size, but then need 235 GB? I'm running Time Machine on a new NAS. Until today, I used to run it on an USB drive. I am reusing the old job using the new 10.8 feature that allows me to add new backup targets. 

When I configure the backup job in the system settings - there are some things like VMs that I want to exclude - Time Machine estimates the backup size to be 109 GB.

When I start the backup, however, it talks about 235 GB. I don't like this, as I don't want to litter the NAS with unnecessary data.

Is it normal that Time Machine shows such a huge difference between the System dialog, and the actual backup?
This discrepancy is odd - most of the time when people see more than a 25 GB error, it's with Time Machine over-estimating the space that will be used rather than under-estimating. You might be on to something where there is abug in calculating things with multiple backup targets - I've not seen that configuration start to misbehave yet - so you might be one of the lucky few to see how things break under that setup.

If you can make this happen repeatedly, I'd submit a bug (or at least poke around) by running the tmdiagnose tool to collect data on the estimates and actualities.

Also, the Backuploupe tool is amazing at getting at space usage and estimate oddities when you use Time Machine. It's not able to catalog both destinations simultaneously yet - but seems to work well enough on Mountain Lion that I still use it regularly.