If you believe that manipulating the tune frequency will give you an edge, think about this.......your threshold plays a much bigger part. Your altering the tune freq because someone says you'll find gold due to that frequency giving you better sensitivity, could come at the cost of losing that gold due to EMI being more round that set of numbers.
Rob Peter to pay Paul......if it gives better sensitivity, it gives better sensitivity all round...EMI....ground noise ect ect. You cant highlight one without highlighting the other.
The R&D that went into the auto tune feature, went into it for a reason. Show me a man that can better pick a quieter frequency in less than a minute using the manual tune feature..... and i'll show you a flying pig.
Coil sizing....
Bigger antenna....up the size of the antenna, and you would expect the tune frequency to account for a stronger frequency if it falls within the EMI level thats being looked at by the auto tune function. The settled frequency/number it rests on, is of a group that was found by the electronics to be the lowest of that set. If you now change directions with your coil, your dilemma can go either way...towards the offending signal, or away from it. Meaning, if there was an advantage occurring from the selected freq, its now compromised by either being slightly less sensitive, or over sensitive for the ground conditions....and level of EMI now present.?
One of the best ways to test the functionality of the auto tune feature, is to place a working PI detector far enough away so thats its being picked up by the one your holding. (very common example when working close by friends )Now adjust your freq manually and see if the old wives tale rings true about sensitivity vs depth.? Effectively, all you'll be doing with the manual tune is.....getting rid of the annoyance caused by working in the same vicinity as another.
Iv been playing with this myth for a very long time, and no one has ever been able to put worth to its validity. And the reason for this is....the amount of variables. Your better off putting the time in prospecting/detecting rather than find any edge from this exercise in my opinion.