You'll hear a variety of answers...
If your goal is to show, I would look up what show judges would consider ideal for tancho.
I have no interest in Tancho whatsoever...but what I have heard is that you want the tancho spot to be in between the eyes and as large as possible without crossing the eyes and head to the shoulder. Many perfect looking tancho are artificial. Breeders and some dealers would "cut" the koi to create the "perfect" tancho pattern. Personally I would take a non perfect tancho over an artificial one anyday. Artificial does not sit well with me.
From an amateur koi keeper's perspective, at least mine, the tancho doesn't matter if it's on a Showa when you know part of it will be "corrected" by the emerging sumi. However, tancho on a Kohaku or Sanke would be more pronounced.