It seems that my Low Rider 1 is cutting straight in the X axis and off by 1/8" over 16" in the Y direction. I squared my Y axis before starting and checked it when the cut was complete and seems ok. I have my Y setup to be in the bridge direction not the rails direction. any hints or suggestions. I thought if my board placement stops were out of square then the X and Y would reflect the angle. but X direction seems straight.
I had the same issue. Check that the wheel axes are parallell on each side. You can read about my adventure in this thread: How to check a populated gantry for square
My did a parallellogram and it was fixed by making that angle bracket and making sure the machine is square when starting up!
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How do you square your lowrider? Do you just pull it against the tensioners? If so, have you checked that your tensioners are square to your direction of travel?
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My procedure is to take my adjustable right angle square ruler and choose one side and adjust from end of the table frame to touch the plate that the axles mount through then go to other side and make it match before turning on the stepper motor power. I thought if my gantry was straight before I start anything the steppers should keep it true. And they did pass my post process testing using the same procedure so i don’t think they lost a lot of steps.
I guess it could be that i have bumped my table frame out of square by accident, it is a small garage shop 
and it’s framed with T 60-80 but could use some more bracing and bracketing. Being unemployed sux for buying parts!
Is the edge of your table perfectly square? I know I had troubles with mine being slightly off using your same method so had to make my stops based on being perfectly perpendicular to the side of the table and not using the back edge at all