I use to install my center grafts on the back prior to installing the braces, but I never was able to get the braces installed without even a slight gap between the brace and the graft. So I changed my method. I know Lance does (or did, maybe still does) it this way and that's where I got the idea.
I now install my center graft after the braces are installed.
Here's how I do it to ensure a nice snug fit.
I first measure the distance from the outside edge of the body to the inside edge of the head and tail blocks. I than mark that distance on the inside of the plate.
Tail
Head
I than use tape to find my precise length of the graft, between each brace. I use the 3M strapping tape as it doesn't stretch at all. So I put tape in between each brace and use a razor blade to cut it to the exact distance between the braces. I than mark each piece of tape so I know which piece goes where.
I than pull the tape off and put it on the center graft. I align one edge of the tape to the edge of the graft (not shown in the picture though as I thought of this after I took the pic) and than I cut right at the end of the tape. This give me a perfect fitting center graft, and actually when gluing them up in the go bar deck, the radius of the back helps push the ends of the graft against the brace.
I make my center grafts 5/8" wide (just because) and here's how I layout each graft location so I get them lined up.
I have a block that is squared up and I put wax on the base and along two edges. I measure from the center of the plate (easy to see with a book match like that Tassie Blackwood) 5/16" (half my center graft width) and I locate my aligning block. I put a go bar on it to hold it in place.
Than I just but up the center graft to the block and the brace and I'm good to do. I have little clamping cauls specific for gluing the graft down.
And a close up of the nice tight fit.
Hope this helps someone.