Generalized, I just add 30% to whatever I see. Sales tax is 8.something% here (and if it's food service, there's often a "Healthy SF" fee of a couple percent), so adding 30% ends up being roughly accurate, or even an over-estimate. I'd rather over-estimate and end up seeing a lower final bill than the opposite.
> it's a legalized way to fool people into becoming customers
Pretty much. People are calibrated to expect certain things at certain quality levels to cost a certain amount. If laws changed and businesses were required to report the final cost of everything on menus, price stickers, etc., that would likely -- at least for some amount of time -- cause patronage to drop. People would get sticker shock and pass on things they'd usually buy, even though the final prices are actually the same.
And of course few to no establishments will buck the trends and include all costs, because then -- at first glance, at least -- they'd appear more expensive than their competitors, even if they're the same.
At least that's how it always goes for me, but the point is, the price you see for entrees in the U.S. is pretty much +$10. If the advertised prices were this high, I wager fewer people would be walking into restaurants.
The industry as a whole is probably interested in this systematic bait-and-switch, and to get from there to the real, "service included" prices would be too much of a shock to the system. The price of being served food is actually higher, and people need to get used to that.
If you think about it, it should really be illegal - it's a legalized way to fool people into becoming customers, but in the United States the interests of a business almost always come before the interests of the public. The only exceptions are extreme dangers to health and safety.