Variables in lists

[Robert F. Buzan]

I'm a fairly green E-Prime programmer and have encountered a problem I can't 
find an answer for.  I'd appreciate any help that anyone can provide.  I'm 
trying to create an experiment that limits the amount of time that 
participants can respond.  It's done in this way:

RT data is collected in the practice phase and the mean and standard deviation 
are computed.  We then create a variable to be used for the allowable response 
duration on one of the later blocks, thusly:  Dura = AveRT - 0.5*stddev.  This 
works just fine.

Later, when I need to use this, I want to set the allowable response time for 
the stimulus to which participants must respond to Dura.  We've tried putting 
Dura directly into the timelimit box, but it must be an integer or an 
attribute.  We've tried putting [Dura] directly into the timelimit box, but 
that is considered an attribute.  We've created a separate attribute, 
[MyTime], and populated that column with Dura (which, I believe, it interprets 
as a string) or with [Dura] (which is then an attribute).  I think I've 
narrowed the problem down to this question:  How do I use a variable in a list?

If that's the wrong question, does anyone have a suggestion for making this 
program limit response time to the value of Dura (Average RT minus one-half 
the standard deviation)?

Thanks for your help.

Gratefully,

Robb Buzan