I have plotted a light curve from a supernova (SN2004DJ) in several
different bands (V, B and R) and have achieved what amounts to a
straight line on a plot of magnitude against time.  This is to be
expected as magnitude is a log scale and the light curve decays
exponentially.  I also want to include on this graph a line showing
the decay rate of Cobalt 56 which is thought to cause supernovae
remnants to radiate long efter they would have otherwise disapeared. 
Whenever I do a calculation to work out the gradient of the Cobalt
line, it comes out to be around 260 days per magnitude.  This is
totally different from the value quoted in a research paper about the
same supernova.  A copy of this paper can be found at:
http://www.scottmarley.co.uk/test/paper.pdf.  This gives it as around
1 magnitude every 100 days.

Can anybody help me to do this calculation?  The half-life of Cobalt
56 is 77.27 days if that helps.

I know this is a very specialised question and might well go in the
Mathematics section as well.  If you happen to be an expert in this
area, please help!  Thank you.