Overshooting my withholding rate's upper limit; invest the extra early in the year or ask for recalculation?

I’m in Finland. The way income tax works here is that my employer withholds a certain percentage of my salary up to an annual upper limit, and a significantly higher percentage of the salary exceeding that.

I got my pay stub today, and discovered that my withholding rate for this year was calculated so that I actually overshot my maximum last month. This just means that I pay a just over double rate for November and December. When I filled in the electronic form to request a new withhold rate for December, the preview tells me I’d be paying approximately the same rate next month even if they recalculate.

So, the taxman is getting his, and the extra 120€ I have been bringing home each month has been going to mutual funds. There’s also been an approximately equal stream to stocks and my rent/insurance/rainy day account. The downside is that I’ll dip into my mutual funds for about 800€ in the next two months to offset the total 1200€ extra withheld in the same time.

My question is: If I get a similarly underestimated withholding rate next year, should I get the rate recalculated or keep doing what I have been (unknowingly) doing this year?