I just tried creating a variable it only lets you assign a value to a dimension. If you need a variable I would suggest adding a reference line in your first sketch and dimensioning the length. Nomatter what equation you add to it it will not affect the model untill you use it in another equation. I was checking this in 2003 not sure about 2004. There may be a more elegant way but this is what I would do until one surfaces.
Corey Scheich