Linear rail not sitting flat

I'm in pursuit of getting a really solid mechanical starting point, I've always just 'solved it in software' before. This time I'm aiming for a sub 0.1mm spread across the bed, if I can get it. I'm edging closer with the creation of some jigs to make minor adjustments, but I think I'm now being hamstrung by a linear rail that just doesn't want to sit flat across the extrusion, as per the bed scan image. My feeling is that the low points are likely a cause of the linear rail not sitting nicely, too. As the image shows, the right (as you look at the front of the printer) Y linear rail now seems reasonably well situated/tuned in terms of the frame. Similarly, both corners of the 'left' Y linear rail are close to the same plane. Any pro tips on resolving this? I'd like to avoid shimming if at all possible. So far, I've been using the standard guide rail jigs, loosening all screws off (except the rear end) and then untightening all the way, before gently tightening down (so as to allow the t-nut to move inside the channel and properly grip), before tightening fully and moving on to the next one. The thought process being that this would prevent any squeezing of the rails between the two end points. Perhaps I should also try starting from the middle and moving out in each direction one by one...