Is there a ruler or something akin to that in Poser7?  What I'm trying to do is have the light (=<) shine on "A", but then stop at "B" so it doesn't hit the wall at "C", below.

=< - - - A - - - - B - - - - C

I'm trying to use the "dist end" dial, but that makes the light start to fade at B so it's just dimmer at C. If I shorten the "dist end" value, then it just starts dimming at A.  I want to get it to stop dead at point B so it doesn't hit point C but will hit A at full intensity.  Will getting the exact distance from the light to "B" and putting that into the "dist end" make that happen?

And a^2 + b^2 = c^2!

You can get the desired effect, but measuring probably won't help much. Be sure to set both Dist Start and Dist End.  A bit of trial and error and some trial renders will get you there fairly quickly.

In this picture, preview is on left and render is on right.  The ball is 'B'
and the wall on right is 'C'.

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Try this tut by Dr Geep:
www.drgeep.com/p4/l1/l1.htm
it's about "dist end" and "dist start". BTW you can find there MANY other tutorials (not only lights).

Another hint: if your scene is complicated, use primitives like these
for quick testing, then copy the light parameters and apply them
to the light in your real scene.

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Ah.....DrGeep.com = fantastic.  Thanks!