So after all my testing i have come to the conclusion that armour upgrades are +2 no matter what the base value was before.
What the armour upgrade is called dosnt matter, its merely a name. Upgrade 23, *gothic* is the same as upgrade 1 *leather* they both give +2 to armour stat.
Armour upgrades are clumulative adding +2 per upgrade to a max of +6, with gold shield upgrade.
This goes for any unit regardless of their base value.
Why the ingame unit card displays +1 i cant explain its a mistake that has stayed in the game from the start. But its really +2.
This knowlagde does not lead me to having ALOT of work cut out for me with redoing and remodding my SS game. As i have units who go from having light mail *base 4* to full plate, where i assumed they went to 11 armour. But no they only go up to 6, if you buy full plate upgrade for them.
Lots of units and tooltips i have to change now. However now i finally understand how the armour system works.
Below is my data from my testing, should anyone want to take a look at it themselfs.
Spoiler Alert, click show to read: