I drew this up for you as a concept, but I haven't built it yet so you might have to tweak it a bit. You'll notice one of the parts has a slot in it so that you can adjust it for squareness.

I drew this up for you as a concept, but I haven't built it yet so you might have to tweak it a bit. You'll notice one of the parts has a slot in it so that you can adjust it for squareness.

Honestly, I'm still a little confused about the issue here ... sorry ...

It sounds to me like the main problem is that your square is not exactly square. If it isn't, then maybe a new square is in order. :shrug:

From another Gulf Coast Guy,

Sounds to me like you have it figured out already. I made three 8' saw guides from one sheet of masonite and some 1/2' ply, and cut one into 5' and 3' lengths. On the 3' I added the 90 deg cleat under it for your crosscut jig, only I did have the benefit of a 24" drafting triangle to square it up with.

Plus, you have already described the adjustment setup and calibration to a tee. I say just go make some sawdust!

Mike

Thanks but that jig I have. What I need is one to make a crosscut at 90 deg. referenced to the straight edge that jig produces, preferably off a single measurement along that edge.

I've seen some on YouTube and elsewhere but they all assume a true 90 off a frameing square or the like. This is used to attach another fence on the underside perpendicular to the cut fence on the top. Once the 90 is established the reference fence is fixed to the jig with screws and glue.

The reference fence is registered to a straight edge of the panel or board and the cut fence sets up the 90 deg. cut.

What I have in mind is one step further. Do the initial setup with the frameing square but just clamp it in place. Drill and insert a dowel at the intersection of the two fences to be used to both anchor the two at that point and to provide a pivot point. Further toward the other end of the reference fence awat from the cut but still under the runner board (didn't know what else to call it) drill a pilot hole to act as a guide for a adjustment screw in the reference fence and a slightly oversized hole in the runner. The runner would be extra wide to have some meat on the side of the cut fence oposite the cut

Calibration:

With the screw holding the jig to the original square from the frameing square, make a cut on a test board. Flip the test board horizontally keeping the reference edge against the reference fence and make a cut on the other side of the test board. Assuming some error in square the length measured at the base of the test board and the top will be different by twice the amount the jig is out of square. Loosen the adjustment screw for a friction contact, move the refernce fence to correct, retighten the screw and repeat the proceedure. When you reach the level of accuracy desired another dowel can be inserted near the screw to lock the jig in place and other screws can be used for reinforcement.

GCG

I'm guessing you considered a good quality drywall T square?
Wal-Board Calibrated T-Square 54 in. - 1/8 in. WTS-54 (WLB-88-016) | WallTools.com

Obviously you wouldn't be able to make the sawcut without damaging the Tee, but they're sure convenient for the layout part.
Unlike a framing sq. the Tee arm is undermounted making for a really solid contact with the reference edge...not prone to slipping off.
I use mine for plywood layout as well as drywall; no complaint.