When restoring to a smaller partition or disk we recommend using the following formula:
(Original partition/disk size * 0.17) + Original used space on the partition/disk = Estimated total needed space on the new partition/disk.
Take into account that this is only an estimate, and that the values are referring to the original disk layout and not the size of the recovery point image.
As an example:
A 68 GB partition or disk having 20 GB of used space would calculate to:
(68 * 0.17) + 20 = 11.56 + 20 = 31.56 GB being smallest space needed for the restore.