2 of 3 added 5 characters in body; edited tags

Roth 401k: tax amounts withheld on contribution and match, how to calculate?

My employer matches dollar for dollar up to 6% in our 401k.

For simplicity's sake, I'll speak in round fictional numbers: So suppose my traditional 401k contribution has been $600/month, and the employer has matched that with $600/month. I now decide I want 5% to go the traditional 401k "side" and 1% to get the Roth 401k treatment. So my next month's check shows $500+$500 going to the regular 401k, and $82+$82 going to the Roth 401k.

Just like my normal withholding, the $18+$18 withheld from that 1% which is now going to the Roth 401k is estimated, not actual. The exact rate to tax at won't be known til year's end.

How does this get corrected? How are shares I buy during the year which are based on these estimated balances modified once the final rate of taxation is determined next year? This seems messy and complicated with lots of opportunity for problems.