Just wanted to show off a cool finding. This is more of how powerful UMD is, and how a Synthesist, and to a lesser extent a Summoner, can gain some huge versatility with UMD.
Here is how I would plan getting my UMD high
1: 9= 1(rank) + 3(trained) +4(cha) +1(guidance)
2: 12= 2(rank) + 3(trained) +4(cha) +2(masterwork item)
3: 15= 3(rank) + 3(trained) +4(cha) +2(masterwork item) +2(skill boosting item) +1(guidance)
4: 24= 4(rank) + 3(trained) +4(cha) +2(masterwork item) +2(skill boosting item) +8(skill evolution) +1(guidance)
5: 27= 5(rank) + 3(trained) +4(cha) +2(masterwork item) +2(skill boosting item) +8(skill evolution) +3(circlet of persuasion)
6: 28= 6(rank) + 3(trained) +4(cha) +2(masterwork item) +2(skill boosting item) +8(skill evolution) +3(circlet of persuasion)
7: 30= 7(rank) + 3(trained) +5(cha) +2(masterwork item) +2(skill boosting item) +8(skill evolution) +3(circlet of persuasion)
... just in ranks, nothing more
Under UMD: Use a scroll, "In addition, casting a spell from a scroll requires a minimum score (10 + spell level) in the appropriate ability" Seeing that all wizard spells are also sorcerer spells, and that all cleric spells are also Oracle spells, you therefore have the Cha to cast them.
Therefore we only need to worry about the UMD 20 + caster level to cast the spell as though the spell were on our spell list
For the following Summoner levels we should have a good chance of casting a spell from a scroll(this is not taking into account the caster level requirements(bolded items), I'll do that in a sec).
3: 1(75%), 2(65%), 3(don't try)
4: 1(95%), 2(95%), 3(95%), 4(90%), 5(80%), 6(70%)...
5: 1(95%), 2(95%), 3(95%), 4(95%), 5(95%), 6(85%), 7(75%), 8(65%)
6: 1(95%), 2(95%), 3(95%), 4(95%), 5(95%), 6(90%), 7(80%), 8(70%)
7: 1(95%), 2(95%), 3(95%), 4(95%), 5(95%), 6(95%), 7(90%), 8(80%), 9(70%)
For sorcerer/oracle spells, because they're delayed a level behind the wizard/cleric, you'll need to have a caster level equivilant to theirs when they could first cast the spell. Just because your UMD check is high enough to make the scroll believe the spell is on your spell list, it does not waive the CL requirement.
For this there's nothing we can do to boost it.
3: 1(100%), 2(95%), 3(85%), 4(75%), 5(65%), 6(55%), 7(45%), 8(35%), 9(25%),
4: 2(100%), 3(90%), 4(80%), 5(70%), 6(60%), 7(50%), 8(40%), 9(30%),
5: 2(100%), 3(95%), 4(85%), 5(75%), 6(65%), 7(55%), 8(45%), 9(35%),
6: 3(100%), 4(90%), 5(80%), 6(70%), 7(60%), 8(50%), 9(40%),
7: 3(100%), 4(95%), 5(85%), 6(75%), 7(65%), 8(55%), 9(45%),
8: 4(100%), 5(90%), 6(80%), 7(70%), 8(60%), 9(50%),
9: 4(100%), 5(95%), 6(85%), 7(75%), 8(65%), 9(55%),
10: 5(100%), 6(90%), 7(80%), 8(70%), 9(60%),
11: 5(100%), 6(95%), 7(85%), 8(75%), 9(65%),
12: 6(100%), 7(90%), 8(80%), 9(70%),
13: 6(100%), 7(95%), 8(85%), 9(75%),
14: 7(100%), 8(90%), 9(80%),
Now if I multiply these together(chance of getting both) we can achieve the chance to cast any sorcerer/oracle spell from a scroll
Level: Spell(Chance%)
3: 1(75%), 2(61.75%), 3(46.75%), 4(33.75%), 5(22.75%), 6(13.75%), 7(6.75%), 8(1.75%), 9(1.25%)
4: 2(95%), 3(85.5%), 4(72%), 5(56%), 6(42%), 7(30%), 8(20%), 9(12%)
5: 2(95%), 3(90.25%), 4(80.75%), 5(71.25%), 6(55.25%), 7(41.25%), 8(29.25%), 9(19.25%)
6: 3(95%), 4(85.5%), 5(76%), 6(63%), 7(48%), 8(35%), 9(24%)
7: 3(95%), 4(90.25%), 5(80.75%), 6(71.25%), 7(58.5%), 8(44%), 9(31.5%)
8: 4(95%), 5(85.5%), 6(76%), 7(66.5%), 8(51%), 9(37.5%)
9: 4(95%), 5(90.25%), 6(80.75%), 7(71.25%), 8(58.5%), 9(44%)
10: 5(95%), 6(85.5%), 7(76%), 8(66.5%), 9(51%)
11: 5(95%), 6(90.25%), 7(80.75%), 8(71.25%), 9(58.5%)
12: 6(95%), 7(85.5%), 8(76%), 9(66.5%)
13: 6(95%), 7(90.25%), 8(80.75%), 9(71.25%)
14: 7(95%), 8(85.5%), 9(76%)
If we follow the 90% chance line we get the following spell level progressions
3:1st
4:2nd
5:3rd
6:3rd
7:4th
8:4th
9:5th
10:5th
11:6th
12:6th
13:7th
14:7th
...
I know that purchasing scrolls is very expensive, but as for having a toolbox of spells like the artificer would, I'd say we're doing pretty good for ourselves.