Area of a triangle = 1/2bh, where b=base, and h=height. 1/2 x 84 x 42 = 1764 square inches.
30% of this area is 529.2 square inches. Solving the equation 1/2bh = 529.2, we get h = 23 inches. However, a delta wing would be violently unstable with its CG at 30% of M.A.C. from leading edge, unless it had a large horizontal stab to help stabilize the delta wing in pitch. The maximum rearward location of the CG would be about 25% back from the leading edge, since the center of aerodynamic pressure will be located at about that point. I would locate the CG at about 22% for initial test flying. If you solve the equation 1/2bh = .22 x 1764 = 388.08, you get h = 19.07. The CG should, in my opinion, be located at about 19 inches from the vertex. You could then very carefully move it back in tiny increments to get the handling that you want, since deltas are, in common with most all-wing configurations, rather sensitive to CG location.

To test CG of unusual configurations, I like to build a simple paper or meat-tray foam hand-launched glider with the same layout. Give it a little dihedral, bend a little bit of "up" elevon into the trailing edge, and experiment with paper clips, or what-have-you for nose weights until you get a stable glide. A simple flat plate airfoil will do just fine for CG checking.