Mold Holder /Side Radiusing

[stringgz301]

Quick question for John. I want to build a mold holder like he uses when radiusing the top and bottom of the sides. I can get the 6" nipple and base and 1/2" dowels etc. What I don't know is where to position the nipple/base. This location is obviously critical since it determines the center of the dome.

Thanks in advance for your help!