If my understanding is correct then you need two rolls:
1 - ESF (Extraordinary Spell Failure check)
2 - the spell casting roll itself.
I’m basing this on the Red Border Spell Law.
For roll 1:
- compare caster’s level versus the spell’s on table 10.9.2. Since sleep is two levels higher than the caster, the ESF modifier is 25.
- is the caster hurrying the spell casting? Is not then it’s a class III spell meaning there will be two rounds of preparation then cast on round 3. Each round hurried increasing ESF by 25.
- are both hands free?
- is the caster wearing at 5+ armor or a helmet?
- how much and what kind of luggage is the caster carrying around?
You add all those mods together and then must roll higher! In your example I can only assume that the other modifier is spell level, so on the third round the caster must beat 25 on an ESF roll.
Failure means rolling a spell failure with +75