Good questions—thank you for starting this conversation.
Your assumptions about testing seem reasonable, and hopefully we'll have confirmatory data soon.
I have with great regret stopped using microCOVID. A factor of 2 - 3 x risk multiplier seems reasonable, but I no longer entirely trust their transmission model. It's probably still more or less valid, but Omicron is a very different disease. There's some interesting data about it preferring the upper respiratory tract to the lungs, and about how Omicron particles behave differently in aerosols, that make me worry that transmission patterns may have changed in ways that are more complicated than a simple multiplier.
I'm hoping to see more data soon (and especially hoping that the microCOVID team will update for Omicron, although that seems somewhat in question).
If you are vaccinated, disregard any advantage microcovid gives to vaccination status. Then adjust all microcovid estimates upward by about 50%. This should give you a risk estimate consistant with new omicron data.
As I assess potential New Year's Eve plans with the intent of spending several days indoors immediately thereafter with my parents (early-to-mid 60s, one has mild-to-moderate asthma but probably healthier than your average 60 year old otherwise [1]), I've been struggling to figure out how to use microCOVID.org given Omicron, boosters, and potential rapid testing.
Here are my working assumptions for everything but testing:
https://docs.google.com/spreadsheets/d/1yZqJKyzPu4CuNFfSR7Tq_QFgSEKrNll-n6iFB-Ly4jw/edit?usp=sharing
For rapid tests, I plan to multiply by a factor of 30% based on Tornus' post citing a 75% reduction in risk from a BinaxNOW test. I cut it down by 5 percentage points arbitrarily to reflect some uncertainty around how sensitivity varies for omicron. Uncertainty comes from the following:
[1] In practice, I plan to navigate this by doing rapid tests every day I am with my parents, and masking indoors with a KN95 for at least the first day. Still figuring out how risky a gathering I am willing to attend given that (hang out with a few people indoors with rapid tests beforehand? Plus masks?).