What equation should I use for microwaving food?
In general, microwave meals are designed to be cooked to a safe temperature throughout, even when a microwave barely puts out the rated power, and that unevenly. This means the meals are designed to handle overcooking. Anyway power ratings are fairly inaccurate as they depend on how well the microwave radiation couples into the food - ratings are based around a defined portion though, so a ready meal isn't as likely to be poorly heated as something smaller.
So one simple approach is to follow the instructions for the highest power stated. This will work well for some things.
You can't assume you can just reduce the time in the same ratio the power is increased, because heat conduction is needed to cook the middle. This is particularly true if starting from frozen, or if it's something you can't stir.
What works for mine (1100W I think) is to use the 900 or 950W instructions, but reduce the time by a little - 30s on a typical single portion that cooks for 5-7 minutes , 1min on bigger things or from frozen, 10s on a dessert that heats up in 40-60s.
Crucially though, this is what I've concluded after trying it - stopping when stuff is bubbling, and checking it's hot through.
If the instructions are for a higher power than you have available, start by following them, then check. It may actually be hot enough but if not, give it a little longer, in the same increments as above (which are only a suggestion). You may find that you consistently need to add the same extra time for the meals you eat, once you've tested