VS 2008 Make A Function That Does What This One Does But Works Faster?

I have a function that I use quite a bit that I wrote more than 2 years ago. It works fine but recently I saw how it can use up a good deal of time under certain condition. Here's some code that uses the function SelectValue The function works so that if you had N which you plug into the function rising continuously while SelectLimit was set to 255 then SelectValue would match N until N reached 256. SelectValue would start to descend to 0 until it reached 0 and then it would again ascend to 255 and the process would repeat. So SelectValue(255, 255) would return 255 and SelectValue(256, 255) would return 254. SelectValue(257, 255) would return 253.

Option Strict On
Public Class Form1
Dim sw As Stopwatch

[code]....

The function works exactly as I want it to but if one of you knows a way to write this so that it will process faster then you can let me know. I get a result for processing time of 6.11 seconds on my computer for the above code.