Formula is :
(1-p)^n<=1-x
lg (1-p)^n <= lg (1-x)
n*lg(1-p) <= lg(1-x)
n >= lg(1-x)/lg(1-x)
You need to heal at least 10 times to score 1 crit with a probabilty of 80% and up.
Since english is not my native language and probability theory and math is pretty complex i cant really explain well how you need to do the maths.