Velocity of propagation (as a velocity factor) in a transmission line is \$ \frac{1}{\sqrt{{\epsilon}_r}} \$ where \$ {\epsilon}_r \$ is the dielectric constant, which for FR-4 pcb material is about 4, so the velocity factor for FR-4 is 0.5, so signals will travel across the PCB at ~150 Mm/s. So 1mm of track length difference is going to be 6.7 ps. Look at your timings and I think you're see that a few mm is going to be neither here nor there. Your half clock time is 8.33 ns, 1250 times shorter than the slew due to a 1mm delay. So you're good, by a long way as far as length matching is concerned.