In Autodesk Inventor, you can quickly name a dimension for parametric modeling by entering "totalWidth = 15in" into the Sketch Dimension toolbox in place of just "15in" - this doesn't work in Fusion. The only way I have found to do this in Fusion is open up the Parameters dialog and manually edit the spreadsheet, which becomes time consuming and tedious!
Attached is a photo of how this feature works in Inventor.