Need help with a domino joint.

joint detail

joint location

I need help laying out the joint of this bed frame with the domino 500. The specific joint is where the rail meets the post. The rail is 1.5"X2.5" (38mmX64mm) and is centered on the 3in (76mm) wide post. I would like to know how to lay it out to get the proper locations for each domino. My first thought would be to plane down two equal thickness pieces of wood to sandwich the rail on either side to center it on the post, then use those to cut the mortise. I want to use 4 ea 8x50mm dominoes per joint. Thanks for any help or direction to a video.