ROCR package finding maximum accuracy and optimal cutoff point

If we use the ROCR package to find the accuracy of a classifier
pred <- prediction(svm.pred, testset[,2])
perf.acc <- performance(pred,"acc")

Do we?find the maximum accuracy?as follows?(is there a simplier way?):

max(perf.acc at x.values[[1]])

Then to find the cutoff point that maximizes the accuracy?do we do the
following?(is there a simpler way):

cutoff.list <- unlist(perf.acc at x.values[[1]])
cutoff.list[which.max(perf.acc at y.values[[1]])]

If the above is correct how is it possible to find the average false
positive and negative rates? from the following
perf.fpr <- performance(pred, "fpr")
perf.fnr <- performance(pred, "fnr")

The dataset that consists of two columns; score and a binary response,
similar to this:
2.5, 0
-1, 0
2, 1
6.3, 1
4.1, 0
3.3, 1


Thanks,
Saeed
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R 2.8.1 Win XP Pro SP2
ROCR package v1.0-2
e1071 v1.5-19