You need to make sure that the first numeral to the right of the decimal point is always displayed. In custom format strings, #
means display the number if it exists, and 0
means always display something, with 0 as the placeholder.
So in your case you will need something like:
=Format(Fields!CUL1.Value, "#,##0.##")
This saying: display 2 DP if they exist, for the non-zero part always display the lowest part, and use , as the grouping separator.
This is how it looks on your data (I've added a large value as well for reference):
If you're not interested in separating thousands, millions, etc, just use #0.##
as Paul-Jan suggested.
The standard docs for Custom Numeric Format Strings are your best reference here.