You are right that there is actually no true flat spot. It is referred to by some hunters as an abbreviation for where the line of sight and pellet path are close enough to take a shot with out holdover or under which is why it is different for different size game. Each hunter sets his rig up for the game hunted and the flat spot. If I zero my 17wsm at 50 yds it is 1/2"high at 100 and I can't hold on a squirrels head in normal hunting ranges. But if I sight in at 1.25" low at 50 yards and take it for yotes that will still give me a lethal hit out to 100 yards and if a squirrel shows up I simple put the crosshairs 3/4 inch above his head and squeeze. I can do this at 1yard all the way out to 90 yards. My scope is mounted 1.75 over bore on this rifle. Now while the 17wsm is way overkill for squirrels it is nice not to have to guess hold over if one hops up on a log at 10 yards. I just hold half a head high and squeeze. I apologize for not making myself more clear when referring to the flat spot. And as far as your scope height of 3.46, with your caliber and velocity it may be beneficial to have the higher scope mount depending on your desired purpose.
