Never worked the weight out first as I have had 3 pt hitch smudgers (Landplanes) before.
Just piled the 3 lengths of rail well back on my rear forks and drove it around to see how well it steered and lifted the weight (MF35 with 3 cyl Perkins).
It is flat land here, but would build it lighter and longer for hilly country.
The Rail is 60 lbs per yard, 2 @ 6' and 1 @ 6' 6'' so about 370 lbs total for rail alone.
But after using it decided it could do with some weight on the rear so added a frame to hold 5 x 30 lb bricks from an old heater. Weight is comfortable when raised but will not go heavier.
The rear blade is now more effective and the hole thing less inclined to tip forward when the front blade digs in hard, note I am using a drop down top link. It is not really practical to lower the lower hitch point as the plane would lift of the ground when the front of the tractor dipped in a hole.
Cannot tilt rear blade down enough, need to cut off rear blade adjuster anchor post and move it forward 90 mm a mistake I made thinking the front and rear blades were exact mirror images.