It's all in the blog post. You will have to figure out 'c' for yourself, since it changes over time. In the post I link to an R script that does it for you - you don't have to do it by hand. Look for "Navigator" in the post.
'c' changed from 300 to 200 about two years ago, but I don't know what it is now.
Edit: I think PostMixer is referring to this post on page 739.
https://bitcointalksearch.org/topic/m.5053668
'c' changed from 300 to 200 about two years ago, but I don't know what it is now.
Edit: I think PostMixer is referring to this post on page 739.
https://bitcointalksearch.org/topic/m.5053668
On the News page of the Slush site, dated 2013-09-25, Slush has posted:
While working on completely new and quite exciting rewarding system, we've changed some settings of the current one in order to improve stability (e.g. decrease variance) in miners' income. It has already been deployed.
The change is about altering shape of used scoring function. The function is now less steep than before which means that your submitted shares influence your score for a longer time period. Therefore your score is less sensitive to instant luck of your miners and leads to a lower variance in results.
I've interpreted it as increasing the c constant for myself at that time. Or might the altering shape of used scoring function mean something more substantial?
. 
I jest!
