Eduardo,
You have requested the entire sweep. There are two ways to retrieve the value at a specific frequency.
Using the trace data you have already requested, you could also request the :TRACe:XSTARt? and :TRACe:XINCrement? which allows you to determine the frequency of each point in the sweep using the equation freq = start + index * increment. Then you just need to find the index into the sweep that is closest to your desired frequency.
Alternatively, you could look at the SCPI marker commands, place a marker at the target frequency, then readback the marker amplitude value.
Both methods would result in the same final result.
Let me know if you have questions.
