NASA has measured it, and it was 93 Dobson Units at its weakest.
Here's an interesting point: in 1958, the lowest point was 110 Dobson Units. If the "record low" is 93, and the low number from 1958 was only 17 points higher, how bad is the so-called "degradation" of the ozone layer?
I tried looking at NASA's site about the "ozone hole" but their site claims that 300 Dobson Units was a normal "low" before the use of chloroflourocarbon compounds (CFCs) became common. It most assuredly is not a normal low, not if the low in 1958 was 110! And please note that this image from NASA's Ozone Hole page does not show the data point from 1958! I have added the data point to show how misleading NASA's graph actually is...and in fact I mistakenly added the point at 120 Dobson Units rather than 110!
I note that the URL for that page shows that the page itself--"history.html"--is in a folder labeled "facts". Where I come from, a "fact" is a piece of information which is not a matter of opinion. I would think that a measured datum such as "ozone concentration low, 1958, 110 Dobson Units" would not be open to interpretation. NASA apparently believes otherwise.
The scientists who first discovered the annual variation of Antarctic ozone--in the mid-1950s--said that the September-October levels were "about 150 units lower than expected". So:
So as you can see, 300 is a normal level, not an "ozone hole" level...and even in the 1950s this was the case. In fact, 300 is higher than the estimated case for 1958!