Fred as promised
It is also on my site at
www.greatsagehabibi.com/ats.php
The dimensions are in metric

The equivalent inches are:
Length = 19"
Width = 10"
Deep end = 4" high
Shallow end = 0.4" high
The rod is placed 7" in from the deep end, I do not think it's vertical position is important though I placed it half way up.
A moveable piece of Acrylic is place 4" from the deep end. This creates an area which fills with water first then flows into the rest of the scrubber. When the scrubber tips it empties, though enough water remains in the partioned of are to allow the bucket to tip back into place.
Since the piece of acrylic is not fixed, it requires enough algae to be growing on the mat to prevent all the water seeping under the partition when it tips. This results is insufficient water to cause the bucket to tip back.
When I started the scrubber I left the partition in place and let it run in the tipped position. Once enough algae was growing it started all by itself. Once it is running harvesting does not effect it. I hold the piece of acrylic inplace with 2 plastic clothes pegs which makes it easy to adjust one starting up.
If I was to do it again I would change a few things, though I have used it for 8 years and has been very reliable.
The main question you asked was the positioning of the mounting bar or rod. From memory once the bucket was built, through BFI (brute force and ignorance) I determined the point at which the weight was evenly distributed. Through sheer luck this has turned out to be the right spot.
I attached the rods to the sides of the bucket with Loctite 480 impact resistant glue, which luckily has not caused me a problem. In hindsight I should have built a mechanism that sat under the bucket. The rods sit in a Warex baring, both the rod and the baring have shown minimal ware over the 8 years.
the bucket is housed in a 24"x12"x20" glass tank, which also doubles as my sump. This way I lose out on the return surge into my tank, but at the time space contraints prevented me from placing the scrubber above the tank.