That could be used as a very rough estimate, but without knowing how much compression deduplication CrashPlan uses it’s hard to know how close to Duplicati needs that would be.
There is a storage provider (destination) comparison here as well as links to some addition offsite detail and comparisons:
Depending on how big your Source data is and how much time / space you have available you could do an initial backup to a local or USB disk just to see how big it would be. You can even adjust compression and other values to be more or less aggressive if you don’t like the resulting size…