how much UMD do you need?

[Karond]

whats the difference between use and cast scroll?

Reliability.

To cast reliably, you need 34 UMD for level 9 scrolls. To use level 9 scrolls, you need 29 UMD. An individual with 29 UMD can cast a level 9 scroll, but he will only succeed on the roll of a 5+ (meaning 80% success rate).

The roll is 25+spell level VS D20+skill.

Premonition is a level 8 spell btw

Also, don't forget that if you're a level 1 wizard, you'll automatically cast all wizard scrolls without fail regardless of level. The same holds true if you're a cleric, druid or whatever. Do note however that some scrolls are labelled wrong, so the correct class can't cast them without UMD from scrolls, while others that shouldn't be able to can.

[Zanniej]

Follow up question on the OP's question:
How much UMD is useful for a bard? Also the same 34 to cast wizard scrolls?

[Deathgrowl]

To cast reliably, you need 34 UMD for level 9 scrolls.

33. You always roll at least 1.