Due to effective compression and deduplication of data, total once-off data usage during the very first backup could be as little as 25% of total data depending on the files. However, you should ideally allow for 100% of total data when backing up for the very first time.
Backups that follow this initial backup though are very small in comparison, as deduplication is highly effective. If a backed up 1MB Excel file has a few numbers changed, only those specific changes are backed up, resulting in only a few KB of data being consumed.