I have made a formula for calculating VS between two waypoints. Here is the formula:
(TARGET ALT-CURRENT ALT)/(LEG TIME/60)
You get the altitude difference by subtracting the altitude you’re trying to get to (TARGET ALT) by the altitude you´re already at (CURRENT ALT). You then divide that number by the leg time (the time, in seconds, it will take to fly between 2 WPTS). Then, to get this number into a smaller number that the equation could accurately use. Just dividing it by the seconds alone will get you a very inaccurate answer. Dividing it by seconds will convert the seconds into minutes. If leg time is something that won’t become a full number, like 170, it will turn your minute number into a decimal number. Then your done! You can also just type the equitation into a calculator.
If you don’t know the exact leg time, and if your FPL was generated by SimBrief, you can download the FPL XML file, open it and scroll down to your WPT. The waypoint list would appear in , and each fix is included in . The name of the waypoint is under . To get the leg time, check under <time_leg>. The leg time will ALWAYS be indicated on the waypoint that you ́re heading to. So if you ́re flying from WPT1 to WPT2, the estimated leg time will be indicated on WPT2.
In the image above, the leg time would be 51
Say you´re at 35000 ft and you have to descend to 33100. The XML datafile says that it will take 1421 seconds to cross. So we subtract 35000 from 33100. Then divide that number by 1421 divided by 60. So our equation will be (33100-35000)/(1421/60). If we put this into a calculator or into a search engine search bar, we will get -80.2251935257. Because Infinite Flight is very picky, we have to round this number to the nearest hundreds place. In this case, it will be -100. If the number is closer to 0 than 100 or -100, just use -100 or 100. If your calculation comes out to be a number < -4000 or > 4000, do NOT use the VS. Use a max VS of 4000.