Fixed Time Trial Bikes

I read this thread every few weeks to get ideas for a fixed TT build and keep coming away with more questions than answers every time!

Job 1 is sorting out the geo of where the bars should be on a road bike. Then I can buy a track frame based on the geo I've established. If conventional wisdom is to have a TT frame a size smaller than a road bike, is the below logic correct?:

1. My road bike has a 58cm top tube
   
2. A potential TT frame would have a top tube of about 55/56cm
   
3. In terms of fore/aft position, the aero bar pads on a TT bike generally sit just above the crossbar of the base bar
   
4. Therefore, on a road bike with a longer top tube, I should start with those pads about an inch behind the cross bar of the (drop) handlebar and play with them from there