I agree. It is best to have 80 dexterity...straight up. Anything less than that, you suffer a hefty penalty.
Your chance of blocking an attack with a shield at 80 dexterity is 35%.
However, once you slip below 80 dexterity, the modifier penalty comes into play. It is a penalty because 80 acts as equal. Above 80 doesn't matter. Below 80 does.
SO...That said, here we are:
1. Straight up calculation of a regular block with dexterity at 60: 29%.
2. Dexterity modifier if > 80. (80 - 60) / 100 = 0.2
3. Then apply the dexterity modifier 29% Base chance * (1 - .2) = 23.2%
So in conclusion the further you get away from that 80 dexterity requirement, the heavier the penalty you are going to suffer. It is best to have 80 to begin with to combat mages who will cast curse and other similar spells.
It really is too easy to do. It does take A LOT of ingenuity on your part to make a suit work. In my opinion, it is one of the more item based templates for a mage in UO. You want your base resists and STR high enough so that if cursed you still stay at or above 100 HP and don't get too low on your fire resist.
Also, for clarification...this is if you had just 30 dexterity.
1. Base chance to block: 20%
2. Dexterity modifier if > 80. (80 - 30) / 100 = 0.5
Then apply the dexterity modifier 20% Base chance * (1 - .5) = 10.0%
*NOTE: All of these calculations assume you carry Legendary Parry. You can imagine how much worse it will be if you don't have 120 Parry.
To be truly effective, you will need to be 120 parry. All good dexxers and archers carry 120 weapon.