Thanks to the mention from @linux535 in this thread[forum], I just took another look at the LessWrong post @dreev just (“just”) linked and whipped up a quick Google Sheet, following its suggestions and z-scoring math. I find the ability to specify separate if-low and if-high \sigma useful in concept, so it supports that by default—if you leave out the high-side \sigma it treats it as the same as the low-side \sigma. The shaded columns (G–N) are not meant to be edited: they automatically fill in based on the data in columns C–F. The confidence-interval breakdown (columns H–K) are to help decide on a \mu and \sigma that feel right. It has the classic 1/2/3-\sigma breakdowns, plus an 80% confidence interval since that’s the range we’d been talking about in this thread.
I put a few small time-estimation predictions in there to spot check the math and will continue to update it if you want to keep an eye on my predictions, I guess? Hopefully it’s a useful jumping-off point for other folks!