So my question is now
if loaded to minimum Sammi spect to replicate factory compatability would i then need to back off my charge as im seating the bullet closer to the powder ?
By that logic i would be off grid so to speak or no long following safe book value?
Or are those starting charge weights accounting for the entire spectrum of sammi min and max coal
I ask because i notice in lee reloading manual there is a Min OAL for each charge which i assume mean going below the OAL makes that charge no longer safe by their calculations
I may be in the minority here, and I don't have the technical background a lot of folks on here do, but I think you're overcomplicating this. If your goal is to load ammo that works in every firearm you have for that chambering, you are necessarily not able to optimize for peak accuracy. That is fine. You may or may not exceed the accuracy of factory ammo, but you can develop a safe load and reasonably accurate load that will function in all of your firearms. My guess is that with proper load development and reloading process you can still beat factory ammo, particularly if you're comparing your handloads to "plinking" grade factory ammo (and not match or premium ammo).
I have never studied a SAMMI drawing when determining how to setup my dies for reloading any cartridge. Every bullet shape is different, so I wouldn't know what to do with the SAMMI drawings even if I looked at them. I'm sure someone does know, but I don't. I recommend you do one (or maybe both) of two things to determine what COAL to use: (1) if you have a manual from the manufacturer of the bullet you are using, use the COAL listed in that manual for that bullet in your cartridge. It will probably work; they listed it for a reason. This will not necessarily work reliably if you are using a manual for Company A and bullets form Company B, however (again, the bullets are shaped differently). (2) As was suggested by someone else earlier in the thread, measure the COAL or CBTO of the bullets you are using in each of the firearms you are loading for, and then load so it fits in the tightest chamber. There are several ways of measuring this and several threads on the topic. If you don't have a manual from the company making your bullet, or they don't list a bullet-specific COAL for some reason, you'll have to use option 2. Even if I have the manual, I'd probably do option 2 anyway just to be safe and compare to what the book says.
With respect to the powder charge, I think you are safe to start a the minimum charge listed in the manual for the bullet you are using. If you don't have the manual for the bullet you are using, and even if you do, it's worth verifying the starting and maximum charges for a given powder and bullet weight and type across at least two sources. Start low (but not too low) and work up. When developing a round for multiple firearms of the same cartridge, I typically do my load development in the tightest chamber, stay off of the max charge, and test the load in all of my firearms before I deem it safe and load a bunch. I've never started with a powder charge below the starting charge listed in a manual.
With respect to "accuracy nodes," I think chasing something like that is a fool's errand if you're loading for multiple firearms, particularly if they are production guns and this is basic plinking or hunting ammo. Find a bullet/powder combination that shoots reasonably well in all of your firearms, and then load for reliability. I think your component selection and reloading practice will determine whether the round is consistently accurate more than trying to vary seating depth or powder charge to find a "node." It's still possible to work up a load tuning powder charge and seating depth to a degree when you're loading for multiple firearms, but you don't have the same flexibility you have when only loading for one, and you have to prove the load in multiple firearms. Again, the apparent object here is reloading good ammo for multiple firearms, not the absolute best ammo for a single match rifle. Different objectives, different methods.