Thanks for the answer. I am trying to figure out the differences caused by adding headset cups in order to figure out the sizing for a new frame. How much do you reckon is added by this - 20mm (top seal is 10mm, and each cup adds 10mm?).

Reason I ask is this - am planning on buying a new frameset (a Lynskey Sportive) and trying to match it to my current road bike (a Ridley Damocles). Looking at the geo, both the ML and the L can be set up fit me. Depending on how much stack height is added by external vs integrated HT (if atleast 30mm), I will decide upon the size.

Thanks!