It's very important to get your trap set in the exact location. If your tailpiece cocks either way you may not get a good seal. Install the strainer in the base and set the base. Stick a pipe through the hole and plumb it both ways and sink into ground. Lift base. recheck plumb, make measurements from plates, set trap with tailpiece and then make your horizontal measurement. Recheck your base before setting it in mortar. Be prepared to redo your drain if it is not right. It can be frustrating.