The whole thing is bullshit if you ask me.
I see job adverts all the time where a company lists exacting requirements of all the qualifications and experience they require, a vague handwavey job description about what the role actually entails, and no mention of salary whatsoever (unless you’re really lucky and they list “negotiable” or “competitive.”) The implication seems to be that as a job seeker you’ll take any old shit and be grateful for it.
You wouldn’t get this sort of cloak-and-dagger dealing anywhere else. You don’t get through the checkout at Tesco with a trolley-load of goods and have the cashier go “well, how much will you give us for that lot?” Then you over-estimate and they profit, or under-estimate and they refuse to sell it.
If you price too high they’ll think you’re too expensive; price too low and they’ll think you’re worthless. Get in the right ballpark and they’ll offer you the role with a salary calculated to be as far under what you asked for as they can get away with without you actually declining. House always wins.
If you absolutely must put a figure on it then I’d work out what you want and add 20% or so, or Google a “what’s the going rate for my job” calculator as they always seem to come out 50% higher than what you’re currently earning anyway. But really, I’d try to get away with something wooly about expecting the market rate and “competitive salary package” advertised and kick the ball back over their side of the net.