Well you should be gold using 100-200 ASA (assuming the Sunny 16 rule) or lower but here goes anyway;
Remember, in photography everything is linked logarithmically. (OK, before you scream, let me tell you, fine arts major here specifically to avoid math, so If I can do it, you can)
Every time you double or halve ASA/ISO you equal 1 stop. 12-25-50-100-200-400 are all 1 stop apart in exposure.
1/2 stop increments would be halfway between those figures.
1/3 ditto, you get it, right?
So if you load 125 ISO film in and want to overexpose it by one stop...?
set your meter near 60 ASA and shoot normal.
If you put a filter in front of your lens and don't know the compensation factor, point the meter at a scene and note the reading. Now, without moving it, put the meter over the cell and note the reading. What fraction of a stop did it absorb?
Dial it in or simply compensate manually; it shouldn't slow you down with a bit of practice and once you start thinking in logs, photography seems to make much more sense.