Although I have not found it mentioned in IRS publications, if the result after the employer adds the extra withholding specified on line 4(c) on Form W-4 is greater the amount available for income tax withholding, I would expect the employer to withhold the entire available amount.
Note that the amount on line 4(c) is specified as an extra amount per pay period, not per month (unless the pay is paid monthly). To determine an equivalent of $50 per month you would need to divide $600 by the number of pay periods in the year. But to have it all withheld for taxes, it seems likely that you would just need to specify an amount of extra withholding that is greater than any amount that you would expect to earn in any pay period.
