One method would be to use a tabular note, create a reference to the attribute in one cell and format the cell accordingly.
Another way would be to reference the attribute expression directly, then you can use the formatting to control the number of decimal places. For example, if the attribute expression is "p0", the note would look something like: "Length: <X0.2@p0>mm". Where the "0.2" represents the formatting (leading zero and 2 decimal places).
Below is from a post I responded to on here several years ago.
<X1.4@p1>
Where p1 is the expression and X1.4 is what controls how the number is displayed.
The "1" (after the X) signifies that the zero before the decimal is displayed (if the number is less than one) - if that number is a "0" then nothing will appear before the decimal point.
The "4" signifies how many digits passed the decimal point will be displayed.
I really didn't know why the difference, so I needed to look it up.
I hope the link will answer your question. I just skimmed it, and it looked good. I will read it more later today.