There's no hard and fast rule (not that I know of anyway). You might get better info posting this on a more caster specific board (Gaffe's maybe? Dunno).
All I've figured out from my experiences is that there is an inate difficulty for every spell in the game. It's based on the level that the "base" class gets the spell at (that should be the class that gets it at the lowest level). There may be some other adjustments as well, since I've certainly noticed that some spells just seem easier/harder to cast then others, even if they are the same type of spell and the same level.
As a general rule of thumb, you should be within about 20 points of the max (5*(level+1) skill that equates to the level of the spell. Unfortunately, that equation doesn't really work at higher levels, but it generally does at lower ones (up to 50, it should work about right. past that it gets dicey). A level 49 spell for example will work best when cast with at least a 230 skill.
Just max your skills out if you can. It's generally worth the time and effort to do so. It's always better to have a higher skill. The equations for casting spells seems to match your skill against the difficulty of the spell, then uses the remainder (if positive) to determine the quality of the result. A negative initial result is a failure (fizzle). Thus, a low skill means that not only will you fizzle alot, but any spells with a variable effect have an increased chance of a lower effect. So your invis spell will fizzle 10 times in a row and only last 30 seconds when they succeed. This effect is one of the big reasons why folks continue to use root spells insteads of the higher level ones in the line. Root is such a low level effect that when you get to a higher level (with a higher skill), you'll pretty much get a maximum effect out of your initial roll every time. Then it's just up to magic resist of the target to determine the final effect. Basically, even though a higher level root may have twice the possible duration, you'll actually average a longe root using the lower level spell.
Nukes work similarly. It's very noticable when moving to a new nuke. With an older nuke, I'll get a max hit virtually every single time. With the new one, I'll get more partial damage hits. Sometimes, it's actually more efficient to continue using an older nuke until you gain a couple levels worth of skill increases (again, this largely goes out the window post 50).
It's also of interest to know that specialization adds into the equation, affecting both fizzle chance and the effect result (more max effects in your specialization area). Um... That doesn't help you as a beastlord, but it's something for pure casters to chew on.
So... Um... Keep working up your skills. It will pay off in the long run. Note that it doesn't matter what level of spell you use to practice. Load up your lowest level spell of the type you want. I remember that beastlords have one type where their first spell is pretty high level and kinda mana intensive. I don't remember which type or what spell, but that's going to be a pain I would think. Unfortunately, there's not much you can do about it. You just need to keep working at it.
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King Nobby wrote:
More words please