It's not about your crth, but the ratio of crth to crtd. Specifically crth should be at a 1:10 ratio to your crtd-100. So if you have 5% crth you should have 150% crtd. At 8% crth it should be 180% crtd and so forth.
To disprove your hypothesis of a crth 20 threshold for locator vs exploiter, compare the following ratios: crth 4 vs crtd 150. And crth 6 vs crtd 150. Compare the additional damage you will get with either a crth 2 mod or a crtd 20 mod.
To extend this, try testing a ratio of crth 5 vs crtd 150. Either mod goes in this instance. Incidentally this kinda proves the exploiter < locator sayings.
what you ignored is, that the ratio is not linear 1:10...at higher crtH, the less amount crtD you need to reach the dmg increase of a single crtH %.
the optimum ratio moves from 1:10 at <10% crtH to 1:5 at around 20% crtH and so on....you can imagine that at 99% crtH the ratio looks more like 10:1 than 1:10.
i'd asume that at 50% crtH the optimum ratio is 1:1