You can spend forever trying to find a reliable source of armour ratings for tanks, then figuring out how much to adjust for sloping and weak points etc.

I don't have that time and the books I have on hand giver very vague and general armour values. 

For History Dad, I basically decided to just rely on the ratings given in the Steel Panthers World War 2 computer game (freely downloadable). Their vehicle stats are pretty well researched and correspond to roughly 1 point of armour per 10mm (with some additions for sloping). 

So the HD combat rules were built to just use those, so if you want to look up a tank that isn't on the tables yet, you can do so.

Gun ratings come from the same place, using the penetration at point blank range (since even a 6x4 foot gaming table is still only going to represent 3-400 metres of distance: Basically knife fight distance for tanks).