Posting comments as answer…
It depends entirely on the structure of the alternative - if the bowl heats up before the contents, then you are dealing with shielding & most 'simple' calculations go right out of the window. Instead invest in higher temperature plastics… These are given away free with supermarket instant microwaveable meals & if looked after will last for decades.
The things they sell microwave meals in are to all intents & purposes 'microwave invisible'. They don't slow the process at all. They're made of HPET if you want to do some health research, but practically I've had some of these things maybe 20 years. They don't really soften with heat & even hot fat doesn't scar them.
I don't buy much microwave food, so I don't pick them up often & then reuse them until I manage to eventually break them. They're great for a can of beans, or leftovers re-heated for tomorrow's lunch etc, or as plate covers if someone gets in late & has to mike their dinner…
& rather irritatingly, even crockery from the same set isn't the same. We have soup-, side- & dinner-plates and cereal bowls from the same set, all ostensibly 'porcelain'. All the plates are microwave invisible, the cereal bowls get red hot without the contents heating much at all.
I once did a fair bit of research on this, when I was trying to find replacements for old, cracked plastic-ware without having to buy the
bloody awful meals they contain, but my research was a dead-end. I had to eat the damn stuff to get the dishes, or buy them by the pallet in 10,000s.
Here's a beginner's guide to plastic types, by 'embossed symbol'. My newer ones class themselves as 'PET PP 05' [& no, I don't know what that really means.]
Hunker - What Are the Numbers That Say That Plastic Is Safe to Use in the Microwave Oven?
Conversely, some of my expensive 'tupperware' [not by brand, just by type of 'permanent long term freezer to microwave or keep food in the fridge] type dishes & bowls are permanently scarred & discoloured if there is any kind of oil or fat on the food being microwaved.