need help scope issue

[tmvikings]

Need help I have a AA24 inch barrel riding a rock river arms lower, also a Nikon Monarch 4-16x42 scope attached by a rock river arms solid piece cantilever scope mount. Today I went to the range first time with it so I started at 50 yards I also should say the scope and mount were on my 223 and it was dead on. I’m a foot low and the elevation is maxed out do I just need to buy a shim for the rail and if so what type. Tried her at 100 and same thing foot low need some ideas on how to fix this. First I thought my the elevation spring was stuck but the recital moves just fine.
What type and size shim?[attachment=1629]