For me the stretchers would make or break the bench, if measured, cut and positioned incorrectly it introduce an instability in the bench and or make it next to unusable.
I wanted them as heavy as possible so they ended up being 100*100mm.
I clamped all the legs to the bench top and placed the stretchers against the legs and marked each on against its corresponding legs.
I used the same mortice gauge to mark the tenons, scribed the shoulders with a knife and went to work cutting the shoulders and cheeks, again I cleaned up the shoulders with a chisel.
The mortices were 80mm deep, 33mm wide, again I hogged out the waste with a 32mm speed bit and cleaned the shoulders with a sharp 2” chisel.
I constantly checked the walls and depth of the mortices with a combination square for depth and squareness, pairing where needed.
I kept the stretchers 75mm of the ground.
I’ve photo documented this build on instagram @joedillon