Got ANY slop between the piece of wood you slip in the miter
  slot and the sides of the miter slot?  If you do it comes
  acrossed in your measuring.  If you don't then the wood
  or even UMHW (or is it UHMW - ultra heigh molecular weight)
  may/will bind in the miter slot as you slide down it to take
  your second measurment.

  Mr. Bennett's product (see how I got around using TS-Aligner?)
  has three bearings under the part that travels down the
  miter slot - two fixed and one moveable left/right to eliminate
  slop while avoiding binding.  And they can be removed when
  not needed - like when checking/setting joiner/planer knives,
  checking infeed and outfeed tables for parallel with the
  joiner/planer cutter head, drill press and router run out,
  . . .

  I've got a Robland X-31 - five function combi with a sliding
  table.  More settings inter relationships than you can shake
  a stick at - and it's a euro machine so there's metric just
  to make things interesting.  The TS-A makes the set up
  and maintenance a bit easier - and every lit bit helps.

  charlie b

  ps - Anyone remember Stuart Brandt's (Whole Earth Catalog)
  bulletin board The Well?  Or the IMSAI? 8080, 8080A or 6502
  ring any bells?  On the first limited production run micro
  processor that actually worked, there are three letters on it.  
  The last letter is an F.  Anyone know what the F stands for?  
  Hint, its the last name of the kid who did the mask drawings.
  What's his last name?