Complicated question.....but I would ask him to chamber it so you can reach the lands and still be able to put rounds in the magazine (Assuming you are building a repeater). In other words, if you give him some one else's OAL, you may find out that you will have to shoot it single shot because your magazine is shorter than the length you provided.
It may not be a problem with a 7 mag/long action, but my 7wsm/short action needed an extended box mag so I can seat my 168 VLD's into the lands and still use the magazine.
If I decide to shoot the 180's I will have to shoot them single shot.....I believe (haven't had that urge, yet).