In April 2013 I used a period where total ozone column (TOC) made a spectacular plunge to calculate the RAF (Radiation Amplification Factor) which tells us by how much the UVI (or biologically effective UVB) will increase when the total ozone column becomes smaller. This month (March 2014) we had a relatively low TOC of 276.5 DU the 7th March, followed the next day by a DU of 328.1. Sky conditions, total solar irradiance and solar angle were practically the same, so that TOC is the only factor influencing UVI. The calculation gives an RAF = 1.17, similar to the value found last year. Broadly speaking, if TOC diminishes by 20%, UVI increases by 20% (here -18.7% for DU and +18.2% for UVI).
See also this paper from last year.
The values used to compute the RAF are the UVI’s, which are proportional to the mMED/h given as red curve in the graph (1000 mMED/h = 25/9 UVI, see here).