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.