If your sure it's 27, and that would be from the nock groove to the front edge of the riser (giveor take a few hairs)at full draw, you'd need a 27" arrow. It should keep any bh in front of your fingers. You can go as far back as the rest will allow, I don't.

I'd customize using any method similar to what Mat said though. Your paying to get them cut anyways. I have someone mark the arrow with a sharpie at where I tell them to do at full draw.

If your not sure your draw will stay the same, get them a little longer.