There are 8 pints in a gallon. 1/6+1/8=4/24+3/24=7/24.
So you need 7/24 of a gallon=7/24*8 pints=7/3 pints, which is 2 1/3 pints.
Another way to work it out is 1/8 gallon is 1 pint. 1/6 gallon is 8/6 pints=4/3 pints=1 1/3 pints. So you need 1 + 1 1/3 pints=2 1/3 pints.
Fluid ounces vary depending on whether you're in the UK or US. There are 20 fl oz to a pint in UK and 16 in US. 1/3 pt = 7 fl oz UK, 5 fl oz US approximately. But the easiest practical way is to use a measuring jug and empty pint bottle. Fill the bottle with water, pour into the jug, and note the measure. Divide this number by 3 and use the result to measure the milk. That will give you 1/3 pint to be added to the two pints.