Your sample file does not quite out to your formula above.
Using your sample - if we assume you want to return Time where 483-533 first exceeds 10 for each SamplePos listed in Sheet2 - and you want to do this efficiently (relatively speaking):
'01-25-11_MDA-WHC.txt'!I3:
=$A3&"^"&($H3>10)
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Then:
Sheet2!B1:
=IFERROR(INDEX('01-25-11_MDA-WHC.txt'!$F:$F,MATCH($A1&"^TRUE",'01-25-11_MDA-WHC.txt'!$I:$I,0)),"Criteria Not Met")
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Performing the same without a helper column will be inefficient - more so the larger the source data set / number of SamplePos being searched for
If you can sort your data you can improve efficiency of approach further.
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