Hi Olivier

From what I have read and gathered from some of our learned forum members, 2 pounds or 900 grams force on the carriage at mid-point seems to be an accepted norm for most typewriters. To get a true reading, the carriage rails need to be clean and correctly lubricated, and the carriage release lever tied down so the carriage is free to slide either way.

The draw tension is then measured with an accurate spring scale or balance holding the carriage at mid point of the travel. Draw tension will increase to the right hand end of the travel as the mainspring gets more tightly wound and less  towards the left hand end of travel as the spring unwinds. (The opposite will hold true if you have a machine that types from right to left, such as a Hebrew typewriter). Hope this points you in the right direction,

Sky