I played around with the game "optionsCustom.txt"
First & Foremost once you put an optionsCustom.txt with some values in it, the game will save those settings in options.txt (at least until you modify something else within the game). This means that deleting optionsCustom.txt is not enough to get back original camera.
Since 2000 seemed almost like cheating to me, I tried to find how to get nearly "horizontal+" view.
math follows :
given the original settings which are :
camera.cam1_trackdist_max = 850
camera.cam2_trackdist_max = 850
camera.dng1_trackdist_max = 710
camera.dng2_trackdist_max = 710
those were designed with 1280x1024 in mind
and we're searching for the camera value for 1680x1050
850 / (1280/1024) = Y / (1680/1050)
Y = 850 / (1280/1024) * (1680/1050)
Y = 850 / (5/4) * (16/10)
Y = 1088
so the closest camera distance for horizontal+ gameplay at 1680x1050 would be 1088 for exteriors
so now let's move on to screenshots:
1280x1024 with default exterior 850 zoom
1680x1050 with default exterior 850 zoom (vert-)
1680x1050 with modified 1088 zoom (approx hor+)
As you can see there's a slight difference between real horizontal+ gameplay and what we get here which is a camera going backward (which modifies the way we see the scene a bit more than what would do real horizontal+, unavoidable as far as my current knowledge of the game goes though but shouldn't be a problem since the camera is freely moveable.)
Now, let's move on to values for each resolution :
for 16/10 resolutions:
camera.cam1_trackdist_max = 1088
camera.cam2_trackdist_max = 1088
camera.dng1_trackdist_max = 909
camera.dng2_trackdist_max = 909
for 16/9 resolutions:
camera.cam1_trackdist_max = 1209
camera.cam2_trackdist_max = 1209
camera.dng1_trackdist_max = 1010
camera.dng2_trackdist_max = 1010
for 5040x1050 :
camera.cam1_trackdist_max = 3264
camera.cam2_trackdist_max = 3264
camera.dng1_trackdist_max = 2726
camera.dng2_trackdist_max = 2726
Conclusion :
for triplehead users, 2000 was probably way too low BUT I don't have the hardware to test it out ...
for the others it was probably a bit too high.