An Average thunderstorm diameter of 12.5 km

An average of 500 cloud to ground lightning strikes /storm ( get these numbers from wiki)

The radius of influence of your mast is about 70 meters.

This simple models than says the probability of getting hit is simply the ratio of the area of influence under your mast to the area of the storm times the 500 strikes/per storm. It comes out to about a 1.5% probability of getting hit if you are a lone sailboat sailing under an active thunderstorm.

Of course, this model neglects a lot of things but it is probably reasonably accurate. I suspect the probability is a bit higher but probably no more than 5%.

So, how much of a chance do you want to take sailing under a storm? I have terrible luck so for me a calculated 1.5% is closer to 15% for most people.