Here is a ballistic sheet ran for my Omega. I use a 6" PBR (max the bullet cam be up or down is 3")... so your calculation of 18 yards might be very close.... If you look in near the middle of the sheet on the right side you can see the calculations for the 25,50, etc...



Here is a 3" computed for a 300 grain Nosler... and it is dang close...