No exact science to this. But growing up, the way I did it was to get a small saucer bowl, spoonful of grape jelly first (because it's easier to wipe off the spoon than PB), 2-3 spoons of PB, stir it all together and slap it on the bread as desired with a knife.
Since then, and having become the one buying the bread for myself, most of the time I get by with straight up PB (Peter Pan Honey Roast is

) or PB and honey as placed on this kind of bread:
and at the very least, warmed in the microwave.
If I feel the need for jelly, well ... if it ain't Smuckers Seedless Strawberry Jam, it's uncivilized. No disrespect to grape jelly lovers, but I'm off that now.
