Depends on how stressfull the situation is... standard is one. But maybe its a storm blowing rain in your eyes? add an extra, and your on now wet rock, add another one, and your running because of that angry troll your bravery having failed you... add another two; oh and your also in an aura strength 4... thats another 4 dice... so take 9 botch dice if you roll that initial '0' O_O
Here are mine with some examples:
1: Casting a fatiguing spontaneous spell in a safe location. Anything else with some risk but in a controlled setting.
2: Casting a spell in a relatively safe location but not under as much control, such as on a raft on a relatively calm river.
3: Attacking or casting a spell in combat.
4: Attacking or casting a spell in combat in poor conditions, such as during a downpour.
it's preparation time and we do not really care (eg preparing a fight, a travel, teleporting to destination) about anything (penetration, time required to successfuly cast the spell without the magus being fatigued); preparation time can occur out of an adventure or between scenes
it's a rituel mastered and is cast outside of any adventure/meaningful time
base is 3 dice (but that's a house rule)
we add vis/any aura value (even magical) number of dices (idem)
special dices (fastcast or whatever)
SG dice, sometimes; generally it's me the SG who does this, 1 or 2. (just RAW)
In our saga, cautious sorcerer and gold cord are very highly regarded.
A magus can choose an extra risk modifier 1-3, this adds botch dice.
The amount of vis seems to have little to do with things, but it does say 'additional botch die for each point in magic aura', and thus for experimental lab work the magic aura counts towards the 'risk'; I suppose thats fair, if your experementing even in a safe aura, your experiment can go extra fluffy
Pages 107-108 core
Assuming an aura of 4.
So first while enchanting he experiments, theres a total of five vis involved in the effect, he rolls a 0, now he rolls one botch dice standard plus a number equal to the level of the aura, or four dice, for a total of five, if one of the five comes up '0' its a botch and time to roll on the disaster chart; his vis are wasted.
...add to this lab safety rules from covenants and your not looking at catastrophic failure that often for a reasonably sane mage.
Edit: I'am reasonably sure I'am forgetting something, it does seem oddly low a chance given how often people actually play with experimentation...
Experimentation is a difficult path indeed.
Experimentation is an entertaining path indeed. The necromancer-Jerbiton in my group has a rain-of-corpses spell where they all look suspiciously like him.
On most forums it tends to be. What tends to be better is to ask your own question in a new thread and reference the old thread when you're asking. At least you were asking a question with regard to a post here. I've recently seen some posts on forums that are trying to answer a question that was asked several years ago, well after everyone has settled the question, which is far worse.
I take from "on most forums" that the position of the community on thread necromancy is similar to the position of the Order of Hermes on necromancy: a not explicitly adressed uncomfortable subject ("do what you want if you don't bring problems to the order").