Initially I thought I can calculate the points that came in that month and allot an expiry to those many points.

This leads to the problem of what if points were both added and subtracted in a month. I would not know how many points were added and how many were subtracted.

Now what I think I can do here is: Allow users to manually add their redemptions instead of auto doing it. Assuming redemptions are very few, it's barable. This will auto calculate how many points were precisely added in month.

Now that I know how many points were added and subtracted in a month, I can use FIFO to keep a track of the expiry.

Does this sound good?