Hi

I asked a similar question a couple of months ago, about bonuses/ 12th & 13th month pay in December, and the effect these had on tax:

First yr in CH - total income pro-rata for tax?

The conclusion seemed to be that tax at source was calculated monthly, using an assumption of multiplying whatever you earned that month by 12 (or 13), whereas tax calculated on a tax return is just done on the whole amount over the relevant tax year.

So on that basis, the tax percentage used in May would be too high. But on the other hand, the one used in Jan-April will be slightly too low (imagine the extra May money spread over 5 months rather than 1). So have a little play with the numbers and see if it's worth your while doing the paperwork!

Hope this helps; good luck with it.

kodokan