We only had to deal with this once, and they were barely sub 18, so it's lucky that your team has sufficient depth. Typically, we'd have a very clear 1 (and sometimes a 2 who's right there as well), a bit of a drop off from 2-3 or sometimes 4, and then a weaker 5.
I think their average times (from late September and on) are important along with placing, with slightly more weight being given to the later meets (i.e. if someone talented but lazy comes in unfit and then gets fitter and surpasses the less talented runner who actually tried).
If the times aren't too distinguishable, could you share what they are or what are expected?