Pre Unearthed Arcana the rules as written are:
Plate Mail by itself, lowers your AC to 3 (Player's Handbook PG 36)
A shield provides -1 to AC lowering the Paladin's AC to 2
The magical bonus (+1) to the shield lowers the AC one more point to AC 1.
You need a Dex of 15+ to realize any adjustment to AC so there is no adjustment for your character.
As mentioned "Full Plate" was brought into the game with Unearthed Arcana. By the adjustments provided by those rules, your original AC -1 calculation is correct.
BUT, the UA also changes the Paladin to a Sub-Class of Cavalier. In doing so, the UA (Page 16) says, "A paladin must have all the requisite ability scores of the cavalier,"
Checking the UA's description of Cavalier it indicates, on Page 14, that "A cavalier must have the minimum ability scores of 15 in Strength, dexterity, and constitution..."
So, technically, you can't have a Paladin following the UA rules with a Dex as low as 11.
But who gets that technical when playing? We just do that in forums :-)