FX Impact Eye Relief ?

Answer = 1.21 gigawatts
jk :)
Actually, the best way I have found to determine eye relief distance is to loosely mount the scope to the rifle. Shoulder the rifle with eyes closed in your most comfortable and natural position and then open your eyes and see if the sight is fussy or clear and then adjust the scope forward or backward until it's clear. Repeat the process starting with your eyes close a few times just to make sure eye relief is spot on for you.
 
I am not an expert, but always thought that eye relief is determined by the scope. The rifle becomes an issue if the scope's eye relief distance cannot be achieved because of the rifle's mounting rail is too short or there are some obstruction, or unable to get a proper scope height, or some other personal preference. I think the Impact has plenty of rail distance to mount most scopes, but as with most bullpups the scope height will be very likely 2.5 inches or more. SethAZ described what would work for your scope (eye relief) in your particular situation.