[Edit: I didn't see Richard's response before posting this, so some of this info is redundant with his]
I'm currently doing a postdoc in Switzerland, so I might be able to help (I got the thesis out of the way close to 2 years ago now ). I was also unsure at the beginning exactly what I would receive and what would be deducted from my pay-cheque, but here is the breakdown as it eventually panned out:
The exact deductions that you pay depend on the canton and even the precise commune (town/village) where you live. I live in Lausanne, in canton Vaud. In total, I receive about 75% of my stated salary each month. The rest goes to taxes (about 11%), unemployment and accident insurance (1% each), old-age & survivors insurance (5%), and pension (8%). The contributions to the last two are matched by your employer, but I'm not sure what the old-age & survivors insurance is all about (in French, it's called "assurance vieillesse et survivants", or AVS) [edit: Richard's response seems to clarify that this is the state-run pension, whereas the other 8% is the private pension. In addition, I also now realize that the 8% for the private pension is only computed on the amount left after the other deductions have been made, so it is actually less than 7% of the full salary before deductions - thanks Richard]
Because you'll have a B-permit and will not be earning a huge amount of money, your tax will be deducted at source ("impôts à la source"), meaning that your employer pays it straight out of your pay-cheque; you do not have the option of keeping the money and then figuring out how much to pay the government at the end of the year. This is not a different or special kind of tax, it just refers to the way that you pay your tax.
You are correct in trying to factor in the compulsory health insurance with your other deductions, even though you pay for health insurance independently. The cost can vary considerably depending mainly on three factors: your age, where you live, and how big of a deductible you want. Apparently, canton Vaud is one of the most expensive for health insurance (I have no idea why), but I am 30 years old and have the biggest deductible possible (2500 CHF), and so it costs me about 220 CHF per month. In other cantons, I believe this could be 30% less. The deductible ("franchise" in French) refers to the amount that you have to pay for by yourself during the calendar year before the health insurance company starts to pay the bills. You can set this as low as 300 CHF, but then you'll pay at least twice as much per month for the insurance.
As far as I'm aware, academics are a lot better paid here than they are in the UK, but your situation may be different. All I know is that you can live a pretty comfortable life as a post-doc here. The pay is certainly WAY better than it is for the equivalent position in Canada (where I did my PhD) even after adjusting for the cost of living.