If you don't plan on checking PRL with bw then starting caber the second or third week of tren dosing should be fine.
Tren a takes a bit to reach peace plasma concentration but caber peaks quickly.
For what it's worth I only use short esters and pin EOD.
When I take caber a take it every four days (every second pin) after my pin.
And follow the same regime with hcg as caber.
Just sharing my experiences of what works best for me.