First off, greek will always be YMMV, it will vary for each girl. Most of the time it's based on the level of comfort and the size of junior.
Second...don't rush it! this hole don't lubricate at all, so take take, be gentle, start by finger or small toys to help loosen it a bit. Don't start by doing deep stroke, it hurt at the beginning trust me. I enjoy greek, but bloody hell those that think that just a bit of lube at the end of the penis will do the job...no!
And you need to change condom EACH TIME you plan on changing hole...the change of getting an infection are REALLY high if you go directly from the rectum to the pussy
Also...if you do greek, please don't try to have the lady give you a BJ after even if it's is bbbj or change the condom without going to the washroom and clean it! it isn't fun trust me!
and except that...enjoy:D