You can get a rough estimation of how safe tap water in a country is by looking at the WHO map detailing the use of safely managed drinking-water sources. Roughly, tap water is OK in countries with >75% of population using safe water sources. This threshold will indeed depend on how certain you choose to be, and the location you stay at. E.g. in decent hotels, tap water will usually be fine even in countries which are just above 50%, because such hotels will have above-average water quality. If you're staying in a sleazy motel on a countryside, it wouldn't be unreasonable to refrain from drinking fresh tap water in countries where less than 90% of water sources are safe.
For my level of risk aversion, filling up a bottle from an hotel sink in Toulouse would be fine.
