First, when I'm tasting, I dilute a sample with water to somewhere in the 10-20% ABV range. That's what works for me. But sometimes I'll dilute with soda, depending on how my tastebuds are running that day.
Second, all heads taste like cheap tequila to me. If I didn't know better, I would swear that the folks selling cheap tequila buy all the heads from other distillers, bottle it up and call it tequila.
Third, for a very generalized -- and I mean generalized -- cutting system, I divide my run into 1/3 heads, 1/3 hearts, and 1/3 tails. Again, this is a very generalized system, but it's a starting point for me to work from.
Fourth, I start my tasting session in the middle of the run and work my way to the edges. If I start off tasting in the heads, my tastebuds are pretty much worthless for the rest of the day.
Fifth, try running at least down to 20% or so. Depending on what I'm running, that's where I find a lot of the flavor. For my plum brandy, it really starts tasting like fruit punch down there. Also, just because something tastes like "wet cardboard" or "dishrag" to your tastebuds today doesn't mean it won't rock your world in a week or two, when mixed with the other stuff and diluted to drinking strength.