The reason I was paying too much tax was because at the time I had an 80% contract. To figure out my tax-at-source rate, my employer for some reason assumed that I had another job with an equal pay rate, with a 20% contract, so they took the tax rate based on the assumption that I'm actually earning a 100% wage, and used this rate on the 80% wage that they were paying me. I've since figured out which forms to fill in to assure them that I don't have another job, and they now use the appropriate tax rate.

Figuring out the right rate of tax-at-source when you have a the 13th month is normally not a problem. My wife gets paid like that and the total tax she pays by the end of the year agrees perfectly with my calculations (and she only has a 80% contract too, but her employer appears to have made the correct assumption that this is her only income).

My advice to everyone is to get the tax-at-source tables from your canton, and figure out the exact amount of tax that you should be paying and compare it to what you are paying. There are multiple reasons why they might differ. If you find this is wrong before the tax year is finished, then you should sort this out with your employer, and I believe that they can adjust the amount of tax you pay in future months to correct it for previous months. But, if the tax year has finished then you need to fill out other forms through the local tax office.

I believe that you can only make a claim for a tax refund when earning under 120k if you believe that the tax-at-source that you have paid is not what it should have been. You cannot do it simply to claim additional deductions (e.g., medical expenses, commuting expenses, etc.). However, I'm no expert, so hopefully someone will correct me if I'm wrong.

I did the sums at the end of last year to see whether we would save any money if we could file a complete tax return. After trying to come up with all of the deductions possible, it appeared that the tax-at-source that we had paid was within a very small margin of what we would pay by filing a complete tax return. I was therefore quite impressed that this simple percentage that they use for tax-at-source works out reasonably accurately, at least for us (but there are so many other variables that are ignored with tax-at-source that I'm sure there are other people who are paying significantly more or less tax than they need to, but can't do much about it unless they earn more than 120k).