Well, you can estimate an approximate level by looking at the bar. 6000 heat means 40% durability cooling elements, which is slightly below half durability. I mean, it's imprecise, but +/- 500 heat doesn't really matter when you're aiming for 6000/10000 heat.
Also, to make sure you get the 40% right, you can simply take a tool with a known amount of uses (like a drill with 150 uses) and use up exactly 60% of its energy reserve. For a drill, that would be 90 uses. The remaining bar is what you should compare the state of your cooling elements to.