Back when I ran around with the XLT, I kept it on about 27 or most all hunting. Whether junky or not. I found that if you tried to go much over that (like into the 30's), you ended up getting a double beep on shallow targets. And while I know the instructions tell you you can go deeper with lower speeds, I'm not so sure I agree with that. I think it's more like you simply HEAR the target longer, hence leading to less-of-a-likliehood you'll miss hearing it (hence the feeling of going "deeper"). But so long as you're adept at hearing quick beeps, then 27 or 28-ish worked for all hunting I did. I never felt I lost depth, yet had better target separation than the stock setting of 20.
JMHO.