Well...I got the roof square...amazed me but I did not have to make any adjustment at all to plywood or shingles...bet it was within an eighth inch over 28 feet....BUT...
It is SO difficult to align old with new. At least for this dumb, retired, former steel mill worker! I have a nice square roof, but the roof does not perfectly align with the base structure....Not a real big deal...but not perfect.
I initially worked off the north-east corner and used the 3:4:5 rule. But somehow I got it wrong, and re did it after I had the first sheets of plywood up.
Even then I have some difference at the south west end. I can do something to make up for it....I don't think it is a big deal...but it is not perfect by any means. Now the roof...well that surprised me. It turned out about as perfect as you could want. Just that due to the connecdting of old to new...well, there is some variation UNDER the roof.
Not sure if this answers your question or not. But if possible, I would advise any and all to build complietely new if possible...