Div adjust to table size

I have an html table with some rows, this table is inside an HTML div, I need this div to always have a scroll.

In order to do this, I am setting this two properties in the css of the div: height: 400px and overflow-y: scroll, yet, as you may know, the scroll only appears when the table exceeds the size of 400px.

Is there anyway to make the div always have a size slightly smaller than the table (for example, for the div to be 90% of the size of the table), or any other way to make the div always have the scroll?