1/8 Black Promist is a workhorse, you can't really go wrong with it. I use it a lot to make modern lenses have more feeling (slight halation) and to take of the edge of sharpness.
If you just want to lower the apparent resolution, you may want to try something like a 1/2 Soft FX, which is another workhorse. It's light enough to remain unnoticed and, if you like the effect, you can always go to the 1/4 or 1# strenghts to adjust it to your taste.
You may also like Glimmerglass (the 1/2# or 1# are quite like the 1/8 BPM in effect) though it's a more evident filter effect than BPM. Som