What if you just created a calculated field and do

AVG(`column`) * 100

Then use that new columns as the summary value 

That is interesting that there is not a "multiply by 100" checkbox in the summary number formatting section like there is when you use a field in the main card area. If that was offered, you could just uncheck that box. Since that isn't option, you should create a calculated field and create the formula like this: 

AVG('ColumnX'/100)

You need to use an aggregate function like AVG for your newly created calculated field to show up in the list of fields when selecting a summary number. You could also use SUM, MIN, or MAX if those functions suit your needs better. Hope this helps.