First off I cut 4 lengths @ 676mm with mitred corners each side. (Inner face is 600mm using CLS timber 63 x 38mm)
I was about 1mm out on one cut but extra not less so cut a water off with my chop saw.
Cut 2 square lengths of 638mm CLS
Mark the centre lines for your joist slots each 200mm across the inner face on 2 of the 676mm lengths.
Then mark 38mm (19mm either side) of your 200mm markers. With a depth of 19mm on the 38mm face.
X that by your 4 markers.
Saw wafers into your joint markings. (The more the easier the clean up with a chisel) you pop the majority out with a simple insert of a chisel into the slot. Then pare away the rest.
Yup looks pretty good on the dry fit.
Use a square to mark the edges of your joins then a ruler from corner to corner to give your central screw hole.
Off to the mill. (Any drill will do here really but a mill/pillar drill will reduce the chances of a wonky entry and blowing a side out your join) that shits not nice.
Some bits ready to assemble.
Fill any shabby bits with filler and sand back.
That's the subfloor at least. (Which will go a good way to support the rest of the framing too)