there is very few 300w psus on the market, most times or are elcheapo brand and will burn your pc or house if you let them thanks to the fact that they are only 150 watts psus
the onlly real 300w psu are those tiny psus for mini itx cases or cases with weird and small sizes
most times what you need is the monimum sold from mopst companies now, a 430 or 450 watts psu
as ypou say, the calculator says that you will use half of that, but that is good , a half used psu doesn't get hot and fan won't be a source of noise on your pc