That... looks like it should work. You sure you haven't missed something different like an include? If not I think I have to try it for myself.
Edit: Nevermind, I just realized what the problem is. onStartTurn is only called for units whose turn it currently is. Use simplayer.onStartTurn instead.
yes, 420 should be the value you want to add. You could also remove a value, by doing something like :
The value could be either a numeric value, like 420 or a value like TUNING.SANITY_SMALL (look at tuning.lua to see the existing value). These are the 2 main ways to set a value, i guess you could have others options (variables ?).