CMES keeps track of the last data measured by placing the info into the D(nn) variables.

-X- value = D10
-Y- value = D20
-Z- value = D30
etc.

You may also create and use R(nn) variables.

R1=1
R2=1.5
etc.

If I understand your problem correctly 
then the code below should help you out.

PT             \! TAKE THE POINT
SP,100         \! SAVE AS POINT 100
R1=D30         \! GET THE -Z- VALUE
R1=R1*-1       \! CHANGE THE SIGN
Z100=R1        \! REPLACE THE + WITH -

NOTE THE EXAMPLE ASSUMES A POSITIVE VALUE TO START WITH < + multiplied by - = NEGATIVE>

Best Regards,
      Joe Corrado

Quality Specialist / CMM Programming
Tower Automotive, Corporation
Granite City, IL