Timeleft In Day for Equation

Hello,

I'm hoping someone could help me try and figure out part of my dataset equation.

I want to calculate the target tonnes per hour based on the hours left in the day and the tonnes already processed.
I have a cap of 13000 tonnes in a day, and production days go from 6am - 6am. 

if hour('*') < 6 then (13000 - tagtot('TAG1','t-18h','*')*24)/TIMELEFT  else  (13000 - tagtot('TAG1','t+6h','*')*24)/TIMELEFT

TAG1 has units of tph

Any help would be appreciated!

Thanks