The simple answer is that a watt is watt whatever produces it - and that a watt is a measure of power, not volume (acoustic pressure level - decibels, that is). The volume is a factor of power AND speaker efficiency (ie: how many decibels a speaker produces for one watt).
As long as we're talking about pure clean tones and not pushing your amp to it's limits, a SS amp and a tube amp set up to produce the same output power (watts) AND going thru the same loudspeaker will produce the same volume (db).
The a-bit-less simple answer is that a SS amp cannot produce much more power than what it's rated for - it will almost immediatly hard-clip your signal - while a tube amp will smoothly go into a more gentle soft-clipping, slightly warming up and compressing your signal but with still quite a lot of "less clean" available power. Given the very dynamic nature of a guitar signal, this means that when pushed to it's nominal power, a tube amp will still have some "not quite clean but pleasantly sounding" headroom where the SS amp will hit a brick wall.
If you factor in the fact that most SS guitar amps are budget practice amps with cheap cabs, cheap and inefficient speakers and more often than not very optimistic nominal power rating you understand where the myth of the "tube watts vs SS watts" comes from.
EQ (from the guitar, the preamp AND the speaker) also plays a huge role when it comes to perceived loudness, specially in a mix where you have to compete with other instruments. Guitar tone is essentially about mids, mids and then mids, so if you end up with a very scooped "V shaped" EQ you'll have to compete with the bass and kick in the lows and with the cymbals in the highs, and chances are you'll need much more power to be heard.
So to make a long story short: no, there's no "chart or rule of thumb", since it's not that much about how many watts but about how these watts are used.