UMD; Scroll and Wand DCs

[PolterGhost]

Wizard, Sorcerer, Rogue with UMD, whichever, I wish to know how to calculate the save DCs on spells cast by scrolls and wands.

DMG states these rules about a scroll:

"The spell must be of the correct type (arcane or divine). Arcane spellcasters (wizards, sorcerers, and bards) can only use scrolls containing arcane spells, and divine spellcasters (clerics, druids, paladins, and rangers) can only use scrolls containing divive spells. (The type of scroll a character creates is also determined by his or her class. For example, clerics create scrolls of divine spells, wizards create scrolls of arcane spells, and so forth.)"

"The user must have the spell on his or her class list (see the spell lists in Chapter 11 of the Player’s Handbook for which classes can cast which spells)."

"The user must have the requisite ability score (for example, Intelligence 15 for a wizard casting a 5th-level spell)."

"Assume the scroll spell’s caster level is always the minimum level required to cast the spell for the character who scribed the scroll (usually twice the spell’s level, minus 1), unless the caster specifically desires otherwise."

For point 4, it states you assume the minimum level required. Does this mean that you can use your own ability score to determine the DC of the spell, that the spell follows the minimum DC from the required Attribute, or that it casts as if there was no Ability Modifier?
That is, if I had 16 Int and I was casting a level 4 spell, would the DC be 17, 16, or 14?
If I have Use Magic Device, do I choose which ability score to use if there are multiple choices?
You can buy a spell scroll at a higher caster level than the default minimum. Can you buy spell scrolls with a DC value higher than the default minimum?
From this statement
"Because they use the wielder’s ability score to set
the save DC for the spell, spells from a staff are often harder to
resist than ones from other magic items, which use the minimum
ability score required to cast the spell."
can one infer that any spell completion or spell trigger item other than Staves use the minimum Ability Score always?

And since Wands lack the special text stating that the spell has to be on its class list in the DMG entry, does this mean anyone can use a Wand of Anything?

 

[FEADIN]

From the SRD:

Spell Completion

This is the activation method for scrolls. A scroll is a spell that is mostly finished. The preparation is done for the caster, so no preparation time is needed beforehand as with normal spellcasting. All that’s left to do is perform the finishing parts of the spellcasting (the final gestures, words, and so on). To use a spell completion item safely, a character must be of high enough level in the right class to cast the spell already. If he can’t already cast the spell, there’s a chance he’ll make a mistake. Activating a spell completion item is a standard action and provokes attacks of opportunity exactly as casting a spell does.

Spell Trigger

Spell trigger activation is similar to spell completion, but it’s even simpler. No gestures or spell finishing is needed, just a special knowledge of spellcasting that an appropriate character would know, and a single word that must be spoken. Anyone with a spell on his or her spell list knows how to use a spell trigger item that stores that spell. (This is the case even for a character who can’t actually cast spells, such as a 3rd-level paladin.) The user must still determine what spell is stored in the item before she can activate it. Activating a spell trigger item is a standard action and does not provoke attacks of opportunity.

Saving Throws Against Magic Item Powers

Magic items produce spells or spell-like effects. For a saving throw against a spell or spell-like effect from a magic item, the DC is 10 + the level of the spell or effect + the ability modifier of the minimum ability score needed to cast that level of spell.
Staffs are an exception to the rule. Treat the saving throw as if the wielder cast the spell, including caster level and all modifiers to save DC.
Most item descriptions give saving throw DCs for various effects, particularly when the effect has no exact spell equivalent (making its level otherwise difficult to determine quickly).

Activation

Wands use the spell trigger activation method.....

Wands are spell trigger items so you need acces to the spell on your spell list to activate it without using UMD.

Magic items always use the minimum ability score to set DCs and level, you can create them with a higher caster level (with extra cost), the only way to set a higher DC that I know (except for a staff), using your own ability as a modifier is by a feat in the Epic level handbook (not an epic feat and not listed in the general feats but on a text page somewhere).
When you take this feat you choose an item creation feat you know an you can apply it when you create an object with said chosen feat (you can take it for each Item creation feat).

 

[Dross]

As you said the Staff entry it says that if better you could use your own ability scores for DCs. This is an exception to the general rule (specific staff ruling over the general spell completion item rules). It is not an example of what everything else uses.

Look at it this way: despite the UMD to activate it, it is not you powering the scroll or wand. You go to the default position of using the minimum required for the spell. Exceptions are mentioned in the item description.