Prepackaged microwave food typically says to microwave on high for n seconds. It usually says that the instructions are meant for an 1100 watt microwave (give or take). My microwave is only 950 watts. What is the equation for me to get the new time? Is it time x (1100 / 950)? 
I'm no plastics expert but a general rule of thumb is the softer the plastic initially, the more likely it is to melt (I know that sound too obvious, but hey;) Something to do with thermoset vs thermoplastic. The trouble with plastics being advertised as microwave-safe is it is a limited claim. If you microwave a bowl of water covered by the lid, then you're fine… forever. However, if you get food, especially fat, on it then that will create a local heat spot far in excess of what the plastic can stand. I've found that it's not worth spending money on heat-proof covers when in fact far more heat-resistant plastics are readily available - for free. Your take-away food will come in one - both container & lid are quite likely to survive being nuked with a really greasy curry inside, with little ill effect except perhaps some staining.  Likewise, those supermarket dishes containing ready meals are even more resilient. I have some of those still in use from probably 20 years ago. They can get a bit bashed & the edges will chip after a time until you decide they're no longer worth keeping.  They all go through the dishwasher with no worries too [so long as you make sure they don't flip over, they're very light.] I have actually been known to buy supermarket ready meal products like this just for the dish ;) 