Confused?
The whole point of Sheldon’s method of determining chain length is to not run it through the rear derailleur at all when measuring. All my bikes (with 2 front chain rings) are set up this way and none can run big-big without a lot of noise/grumble, some will explode the rear derailleur if I tried (long cage derailleurs).
If you used Sheldon’s method for a single front chainring set-up you’d end up with too short a chain (see below)
No, I’m not confused at all, perhaps you misread my post.
And no, my road bike, with stock chain length, is coincidentally as Sheldon prescribed, and the RD looks nothing like that. I’ll take a pic if you want.
And when it’s big at front, small at rear, it’s exactly as Shimano specify here:
Whether single on the front or not makes no difference.
We could go back and forward all day (and night) 
There’s no way I could do the above on my big/big combo’s … not sure if it’s chainstay length, cog/ring sizes, derailleur type or a combo of all of them. Just sayyin’ … might work for you but it comes close to, or does jam if I try to run it that way.
The whole principle behind Sheldon’s chain length method is the assumption that you won’t be running big/big and small/small. That you can doesn’t mean that every bike can do the same. I will concede that it may be possible on some bike set-up’s as you’ve shown above.
Thinking: Most of my bikes have the B-position screws wound right out so as to avoid the pulley’s hitting the largest cogs. Because of this there’s no way to run the big-big combo’s.
I just like arguing. This is me:
You’re not alone !!!
But I’m also trying to learn/make sense of this as well. You’re a smart guy and I know you wouldn’t be sticking at this unless you were right. And I’m trying to work out why it can’t work on my bikes like it does for you if we’re using the same method of determining chain length.
UPDATE:
Turns out the Cassette was fucked. Bought a new one just to try and all my troubles were gone!. Also got her a Paul Chain Keeper for peace of mind and It’s all running really well. She rides the shit out of that thing. Loves it, I’ll post some photos.
Thanks again for all yous helps
The problems could be caused from a few different things.
Maybe your rear derailleur hanger is slightly bent, maybe your rear derailleur has a kink in the cage, maybe the cage on the rear derailleur is either too short or too long, maybe your cassette is worn, maybe your chain is quite loose and worn, maybe your chain is not the right length, maybe the rear derailleur has worn springs that reduce the pull on the chain causing lower than normal chain tension, maybe your BB is too short or too long. There are loads of different causes for these types of problems.
I have sent you a PM although it seems that you have the problem fixed.
Happy riding