First of all, you do not need to put % in front and behind your variable names. In fact, you shouldn't do that. It will just confuse you later. That is how you designate a variable when you want to type it in a field. So, you would have to type %%kettle-on-time%%.
Also, you have not created a GV connector for this variable yet. So, what device did you add to the dashboard? You have to create a GV connector is you want to manipulate a variable on the dashboard.
I would suggest reading through this post on GV connectors and how they work in general. This will be a big help.
