The Sectional Appendix currently give the location of the old Armitage Jn as the split, which also marked the then end of Rugby ROC control area.
The next stretch was controlled from Stoke, but that has now also migrated to Rugby, whose control now goes to (I think) Betley Road, south of Crewe.
Signalling changes at Crewe are in hand which will change things again.
In the end the boundary will probably be that between Rugby and Manchester ROC control areas, wherever that turns out to be.
At least there will be no HS2 complications now (!)
The half-way point on the WCML at 200 miles is close to Euxton Jn, but then Scotland is separate so half-way to Gretna is just south of Crewe.
Not to forget that NR's Eastern Region starts as soon as you turn off the WCML to the east at Stoke or Nuneaton.
And the "Western" starts within shouting distance of Crewe*, towards Shrewsbury.
* actually at Willaston, MP 2.60, beyond which is controlled 135 rail miles away, from Cardiff.