studly,
i can sypathsize with you since we don't live in a place where brisket is ez to get either. since you are locked into doing flats my bet is with enough practice you will soon be the group's expert on flats since you for sure aren't giving up.
ok, the thing you are after is slicable for your preference. why not give this a try as i think it might work.
put your temp probe in the middle. just eyeball the thickest part and the thinnest and put the probe into what looks like the median thickness. i would cook that to 185 take out and foil and let it rest. cut it, chew it and write the results down. that will give you a starting point of sorts. one other thing i might recommend is to reduce your cooking temp around 20 degrees but i think you are better off using your standard temp on the 1st trial so you only have one variable you are working with.
one technique you can use to trick the mind is to slice it really thin if it is too tough as the perceived tenderness will be affected by the slice thickness. no reason to waste good brisket.
just take good notes. make only one change at a time and i'll bet within your 3rd try you are going to be really close to what you want.
good luck
and keep figthing the good fight!!!!
jack