I back in until my trailer's fender is submerged except for the highest point which is right at the surface. In other words, I back in until the trailer's front tire is completely submerged plus 2 or 3 inches.
This works for me at ramps that are steep, average, or shallow slope (although at shallow ramps, the back tires of the vehicle might be right to the water).
I mostly power-on if it doesn't seem to be an environmental concern at that ramp. Otherwise I winch it up.
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings