I did a quick check over our wifi_measure data. The average of the last million rows is -84.56, the median is -85. So -80 means we treat unknown as "better than usual". I would rather treat them as worse than usual. The max/min values we see are in the 0 to -255 range, so not useful as a guide. I tried estimating a 10/90/99 percentile value via a variation of: `select `signal` from (select `signal` from wifi_measure order by id desc limit 100000) as t order by `signal` limit 10 offset 10000;` which results in -70 / -94 / -98.

So I'd be tempted to use -100 as the unknown value.