Only the person sending the merits can determine how to allocate the merits. With how subjective quality is, everyone here may have different views on what is high quality and what's normal. Trying to automate the system can lead to issues with users asking why their post, which they consider to be high quality, did not get theexpected amount of merits.
You just have to try working effectively with the amount of source merits available.
But do sourced merit actually do expire??
Yes, unused sourse merits expire at the end of the month.