There was a misunderstanding, she'll only be able to do this in the start of 1234... She's too cramped by Autumn 1233 
Well, the roll is based on her Casting Total, so, according to the corebook, unless she is stressed, she should roll a simple die.
Since it bothers me to roll dice everytime just for a bonus, I've skipped it entirely.
But well, if you want more spiders, sure! 
It could very well mean I extra spider, after all 