It can be done by using a field in place of the dimension text. When you insert the field, select "Objects" as the field category & select "Object" in the field names. Select the object that you are dimensioning & select "length" as the property.
The trick now to do the math is to highlight & copy the text & the bottom of the window in the "Field Expression" box, change the field name from Object to Formula. Right click & paste what you copied into the "Formula" box & type "- 5" (without quotes) to complete the formula. If you click on Evaluate you will see the result in the Preview box at the top. Change the Format of the field to get the data to appear the way you want it.
It is a few steps but it will get you the results you want.