Currently you can only dimension off intersection points in a .dwg however this is clunky. In an ipt environment you must create "apparent intersection" points first before you can do a dimension off them. Below is a video of me using an old software called Key Creator. As you can see it is much quicker to dimension off intersection points using this software. Inventor should use a much more intuitive environment such as this.
Kubotek KeyCreator 7.5.2 - [TAI-1189_SLSDC30400 R1 DOORS WINDOWS PORTLIGHTS_08-31-18.ckd [Part1] _] 2018-10-25 9_34_16 AM.mp4