This is a highly variable number, you cannot calculate it. The Windows memory manager constantly swaps page in and out of RAM. TaskMgr.exe gets it from a performance counter. You can get the same number like this:
using System;
using System.Diagnostics;
class Program {
static void Main(string[] args) {
string prcName = Process.GetCurrentProcess().ProcessName;
var counter = new PerformanceCounter("Process", "Working Set - Private", prcName);
Console.WriteLine("{0}K", counter.RawValue / 1024);
Console.ReadLine();
}
}
Do beware that the number really doesn't mean much, it will drop when other processes get started and compete for RAM.