Dan:
If you're housing the two braces with 1/2" housings and the tenon is 3" long then there will be a gap between the two tenon ends when you put them into the through mortise created for both tenons.
These means they should hopefully never touch.


You have shown a 8x8 hemlock plate. And usually I like to make the overlap of the scarf 3 times the depth. So that would be a 24" overlap. Jack Sobon told me once he likes to use a 4 times depth overlap. That would make it 32", and a longer slope.

One way I figure the location is by the length of the timber.

Like this:



Now you can see that the long piece of the plate will fit into a 16' timber which I would imagine you can locally acquire. And that the short piece of the scarf will fit into a 10' piece.
10' + 16' = 26' less 2' for scarf = 24' leaving 6" off extra length to trim off to get a 23' 6" plate.

After your scarfs are cut and the timber is pulled up (joined by the scarf) and ready you then layout your post and braces locations.

If you're doing a 3' brace layout you should be ok, but I haven't drawing it exactly to be sure.
You can always make the center two braces a little shorter in the layout if you feel that the brace is too close to the end of the scarf.

Hope that helps.