If what you want to do is to create a circular buffer of a particular size, there are two ways I can think of to do this, one "easy" and one "hard".
The "hard" way is to (a) create an array (Initialize Array) the size of your buffer, set your "input pointer" to 0, then as values come in (one at a time, in this example), replace the "index pointer" value with the new value, and increment the index pointer modulo the array size (do this in two steps -- the first is a simple Increment, the second is the Quotient/Remainder function, with the Array Size as the divisor). Keep all of these things in a Shift Register, of course.
That puts data into the array. To take data out of the array, you simply need another pointer (which you also increment modulo the Array Size) and use it to Index Array. You might want to test that your Output Pointer never passes your Input Pointer (if it catches up, then you've read and processed all of the points).
The "easy" way to do this is to use a Lossy Queue (which I'm sure NI implements as a Circular Buffer internally). Create the Queue having the size of your buffer. As data come in, simply do a Lossy Enqueue Element, and to get the data out, simply Dequeue Element. The Dequeue will "block" when all of the elements have been processed, and the Lossy Enqueue will simply overwrite older elements (including elements that haven't yet been processed!). If your data can fit the Queue model, this is much the simpler (and less error-prone) way to do this.
Bob Schor
