There are ways around it but it seems most people go with "my base was XX.... and that's one of the reasons I'm out there looking to change jobs."
Are you referring to Japan?
I've worked in two states in the USA and never been required to produce such documentation.
You can give them sufficient detail without handing over your P45 or income data. I have never given a P45 to a new employer and it has never been a problem.
It's possible that your tax code will reveal some information, if you're still in the same financial year, but this is an unreliable way of calculating salary.
Do you mean in Japan or the US? Because you certainly don't have to do that in the US.
> From there it's trivial to calculate your previous salary.
And even if you did do it in the US it would be impossible to calculate your salary from it because it depends on how many exemptions you claimed in your W4 (which you can change at any time - it doesn't have to be how many children you have, it's basically a number to fine tune how much tax you pay so you don't overpay and need a refund).