The variables are used to quickly and easily associate multiple nets/pin-pairs etc together in a similar way to match groups but providing more scope of flexibility. I think that it is probably best to use an example. If I have three nets A,B,C and I add the formula to them using the variable MatchMe then I would see...
In this example if
We obviously break the requirement but the question is which one of the nets do we hightlight in error?
The algorithm is such that it simply trys to solve all the formulas using a value for the variable MatchMe and then highlights those which are outside the tolerance.
When we have the statement of A=MatchMe we are not setting MatchMe to equal A; what we are setting is that A equals the variable MatchMe and MatchMe will be determined by solving the formula for all nets that use this variable.
So in your original example where you have A=SomeLength and then B=5000-SomeLength+/-10 it is not setting SomeLength = to the length of A.
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Try leaving out the equals sign when entering formulas.
Here's another take on matching lengths (from a recent post elsewhere):
Try this to match length of electrical net to pin pair - image references shown in ():
- Set Min and Max Length for the pin pair (Net D)
- Enter a Match variable (PINPAIR) and Tolerance (0.5) for the pin pair (Net D)
- Enter the Match variable (PINPAIR) for electrical net (A^^^)
- Configure the autorouter passes as shown and click "Route" (autoroute result may be improved with addition of Route Obstruct)
- Update actuals in CES and review