i don't know one way or the other, but when i look at the black level test pattern, if i can see 17, 18 flash, then i don't think you need to compensate for ambient light hitting the screen. and when i turn every light to 100%, i can still see those levels flash.
my understanding(which could be wrong) is that the bt1886 compensates for 'black' not being black. if you have a higher MLL, it's almost like it crushes blacks. instead of black being .00ftl, and 5% being .1ftl(or whatever it is) you end up with black being .09 and 5% is still .1, so there's almost no difference between the two, and all the detail in 5% and under is completely lost.
if you have a black level that's stupidly high(like what you get when you measure the light reflected off the screen) it's going to go crazy trying to compensate for this. probably to an error, if i had to guess.
bottom line, i would, and have used contact mode, and simply aimed for a higher peak luminance. then, i watch content, and since it looked good, i left it alone.
but i am certainly no expert. i just made a different assumption than you, and it seems to have worked out.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1130K, 7.1/5.1.2 audio
Sources: HTPC(Enby), PS3, XBOX360, Wii
Control: Harmony One