Don't replicate the actual module layouts...just the signal flow!
If you look at the ARP 2600, you'll see a pattern: sources are up and leftward, VCF and VCA up and more middle, mixer up and right. MOST modulation sources are down, with the exception of the two EGs.
Now, what this does is to set up a signal flow in which modulation and CVs come UP to the sources and modifiers. Then the audio from these goes ACROSS from left to right. Now, this is where it stops as the output on the 2600 is right above the mixer...but in a Eurorack build, it's possible to make the flow even clearer by following the ARP 2600 pattern and THEN "correcting" for the different format. So, in Eurorack, you'd want your sources (VCOs, external in, etc) in the upper row and left then toward the center. Sum these (and waveshape them, ring mod, etc etc) at the right end and send the audio DOWN to row 2, where you have your filters and effects, then DOWN once more to a performance mixer and out. Meanwhile, your control/timing should be in the lower row, next to the mixer (which is technically also a controller when you think about it!) and its signals should feed UP. Above that row are modulation sources...LFOs, EGs, noise and S&H, etc...so that they can "branch" out from that area to affect the surrounding modules as well as tamper with the upward-moving CV/mod signals heading to the sources above them.
Now THAT is how you lay one of these out. Everything is in "blocks", so that when you need to adjust something which you hear is wrong, you can more instinctively go to the area where the problem is coming from. And the flow makes sense: up-left, down-right.