Considering the 7mm slimlines using the standard finial, I try to match the diameter of the casing and make a sharp rounded taper at the finial end. I don't like the longer taper that goes straight from the casing to the finial. I've also found that different clips look different too -- the ball clips (like on the 7mm designer series) seem to fit best; the straight clips don't fit as well, and seem to have too narrow of a gap between the clip and pen body. The last bullet pen I made, I used the insert from a 7mm European into the 7mm tube, and threaded the flat top finial from the 8mm American kit - the body diameter was almost the same at both the casing end and the flat top finial end.
Here's one with the std 7mm finial and straight clip - you can see how close the clip gets to the body:
Here are a few more with different clips and profiles - the deer head and rifle clips also get close to the body, but the ball style clips stand proud:
Here's one with the flat top American finial and clip: