A sum doesn't really work, yeah. You need to have a reasonably high number on the low end so that small fleets - which critically includes the player's starting fleet - aren't running blind. You also need a reasonably low number on the high end, so that stealth is possible. E.G. if you had a sum, then even the multiplier from "Go Dark" wouldn't be enough when there's a large fleet present. Also, it'd be kind of weird if the best thing for exploration was lugging 30 capitals.
The alternative way of doing a sum is to have a higher flat value and smaller contributions from individual ships. This works a bit better as far as producing the right range of values, but makes individual ships not matter very much, i.e. the difference between having a capital ship and a frigate would not be pronounced at all.
Counting the top 5 is an in-between solution - only the "best"/"worst" ships in your fleet matter, but they do actually matter. And the range of values produced works.
That said, though - I wonder if HRS could maybe provide a bonus to sensor range regardless of whether it's in the top 5 or not, similarly to how the Salvage Gantry works, with diminishing returns. And then phase ships might do the opposite to sensor profile. That way some ships could still be special enough to matter outside the top 5, but in a constrained enough way. Hmm. Might be worth thinking about.