Unfortunately, at this time, custom DKP columns are not displayed with the in-game mod.
This is something that is slated to be done, but it's just not available right now I'm afraid.
Might I ask why you're subtracting Decay Rate from the Total Earned?
I want the players total to eventually come back down to zero. Currently the system is only decaying the amount of points spent.
So as it stands a player can raid earn 1000 dkp and spend 500 which will give them a dkp value of 2 but then can stop raiding all together and eventually come back with a dkp value of 1000. The system is doing this earned/(spent-decay)=total
It should be doing (earned-decay)/(spent-decay)=total