1mg every other day.
Pill cutters are a load of crap, they never work very well, especially on a small pill like proscar.
I use a surgical scalpel which you can buy at most art/craft shops.
If you are right handed:
Take your scalpel blade (no need to buy the handle) in your right hand
place it on the pill where you would like to cut
put your left index finger (the one next to your thumb) on the back of the blade
and with this finger (index on the the left hand)
slowly push down,
the pill will split, and nothing will shoot off as the soft fleshy 'finger print' area of your left index finger will grip the pill.
Flat edged '11' surgical scalpel blade
I use a SWANN MORTON 11 or 10A blade, many will work, try not to get one with a rounded or curved blade, 11 and 10A have flat/straight blades.
For the dexterous only:
How to chop Proscar into equal quarters.
You can also buy digital scales cheaply that are accurate down to 1/1000th of a gram, and are not too expensive, these should help you divide your Proscar with increasing accuracy.
Good Luck