Hello,
 
  I will try in this post to summerize tips about "placing markers/locators" for a good calibration.  
 
Locator: is the 3D point representing a feature (corner of a house, corner of a window, ...)
Marker: is a 2D point representing a locator in one image.
A locator is defined by several markers in different images.
 
  TODO: for "good" calibration markers/locators the general rules are:  
- cover your 3D scene (not all in the same plane)  
- cover your image (not all centered, or not all in a restricted area)  
- be sure of the position of your locator (Avoid guessing positions !)
- put markers in all images where they are seen.
- select first features that are seen in many (all?) images.
- calibration does not start with less than 8 locators.
 
  Shortcuts:
- Delete : erases the selected locator
- Backspace : erases the selected markers only in the active view.
- Ctrl + left mouse : force creation of new locator (green mouse cursor)
 
This post can evoluate according to your feedbacks, experiences and questions.
 
Stef.

.... may I add... also before shooting your pictures, make sure there will be enough places good to mark... otherwise, cut small color stickers (tiny ones) and stick them in places you know you'll need a marker...
 
as always, all care, no responsabilities...
 
Cheers, Hervé

Please send me the camera model in perspective view of the calibration procedure in ImageModeler, specifically I need the matrix of calibration with intrinsec parameter.  
 
Thank for all
 
Diego