You seem to be basing your calculations on the 530 chain instead of the 10B [ 5/8 x 3/8 British Std ] the bike was designed for.
The new IWIS chains are the same dimensions as 10B. Mine has lasted so far, 4 times longer than any other chain I have used and I have adjusted it slightly only twice.
So many people have been reinventing the wheel on these bikes. Some great improvements and others just money and time wasters. I am a retired auto engineer, And was taught to fix problems, rather than just fit parts.
Before I knew much about Commando's, in the mid 70's I worked out a few modifications to mine which have made the bike a lot better.
1/ throwing away the useless side stand on my 71- 750.
2/ Making up and hard chroming a new swing arm "kingpin" and winding it into the g/box frame with a tool designed for mini suspensions. That pin is so tight, it will never come loose.
3/ Work out a gravity feed for the 90 oil, to the swing arm.
4/ Reinvent the centre stand so it could never break. [ its built now like a brick shithouse ].
5/ Reinvent the pivoting system for the centre stand so it cant flog out the g/box frame, and never work loose.
5/ about 5-6 years ago, and before I knew about this forum, I had made up new cushion pads for the isolastic's on both my bikes. They are a white plasticy substance somewhat similar it seems to the stuff Ludwig used on his extra centering devices and the "Ludwig Head Steady "
6/ I made up my own Ludwig head steady.
7/ Fitted an Iwis rear chain.
My observation, is that the simplest modification which is solid and unbreakable is the best solution. Other head steadies on the market are expensive and complicated. The engineers who make them would probably be better of making other specialized parts rather than waste their valuable time making things like that, but that's up to them. They work, and while rich buggers out there with more money than sense buy them, I say good luck to them and good business..
As an aside, I rode the length of the South Island of NZ, without a head steady, and not knowing it wasn't attached. I have mentioned this before, while riding through St Arnaud, I heard a light metallic rattle, but could not ID what it was, and the bike performed normally. I arrived in Dunedin for the National Rally, and did the ride the next day up to Palmerston, on to Middlemarch and then back down to Dunedin via Mosgiel, following John on his 500 SINGLE and Murray on his Mk III. The next day was shite weather, rain rain rain. We then attempted to leave Dunedin on the Monday Morning after a very rainy night, and found that a lot of bikes would not start easily, including mine. Mine turned out to be a faulty ignition switch, but in the process of looking, found the bolts to my head steady had disappeared. After getting the bike operating, I found the Bill Veitch had some BSF bolts so went to see him in Mosgiel. He got longer bolts and cut them short enough to just about bottom in the head when they were tightened. They have never come loose. A big thanks to Bill, and when I offered to pay for them etc. He would not hear of it.
I can only suggest that because I had those white plasticy cushions set to 5 though gap, in my isolastic's, the bike still handled normally. I don't normally ride fast but the ride from Middlemarch did prove how well the old 500 could go pretty fast, and i kept up with them without any effort;. Steep winding sweepers all the way up and down.
I experience a broken head steady on my 750 way back in 1974 . The bike was like riding a rubber band. Bloody awful, so I know what I should have felt on the 850 ]
After the Dunedin ride, I stripped and cleaned the 42 year old ignition switch. It was caked with grey graphitey looking dry crud. After cleaning, the contacts looked like they were brand new. Its back on and still working perfectly.
Happy New Year.
Dereck