You'll notice that all these are linear systems, which means a texel
rectangle size is independant of its position. So you can get your
vector once by computing the position differences between (0, 0),
(0.5, 0) and (0, 0.5) for instance. You'll notice the equations
simplify nicely when computing deltas if you do the math.

I suspect it's not your real problem though, so feel free to correct
it :-)

OG.