We measured at meteoLCD a spectacular decrease of the thickness of the ozone layer between the 22 and 23 April 2013, from 382 wdown to 266 DU in one day! As expected, the UVB irradiance (and the UVI) jumped by quite a lot. I wrote a small paper on this phenomen, and used the occasion to compute the important Radiation Ampliflication Factor (RAF), probably the first such calculation done here in Luxembourg. The result is: RAF ~1.1, i.e. close to 1.
So a quick answer to the question: “by how much will the UVI increase if the total ozone column drops from highTOC to lowTOC is to apply a multiplier highTOC/lowTOC.
Here a picture of the measurements at Uccle showing the TOC plunge: