The toe span method of sexing Marsh Harrier chicks in the nest in order to determine the correct ring size
 
Is determined  using the toe span measurement and the tables in the book. 
Handleiding veldonderzoek Roofvogels, Guidelines for fieldstudies on Raptors KNNV Uitgeverij,- by Rob Bijlsma.
The relevant pages are contained in the embedded pdf document double click on it to access .

  

Toe span is measured from the tip of the toe of the front middle talon to the tip of the toe of the hind talon excluding the claws. See photos below.

Two people are required to take the toe span measurement.

If two people are not available then the tarsus measurement taken laterally ie from front of foot to back of foot is an absolute must as females will have a tarsus of over 9mm.

Occasionally it will be found that if both toe span and tarsus are taken although the toe span measurement = 80mm or more, but the tarsus does not exceed 9mm. In these cases if the toe span measurement had only been used on its own it will never result in using a ring which is too small!
Page 138 in the tables shows the relationship between number of days old and winglength
Page 139  in the tables shows the relationship between number of days old, weight and toe span		   The measurement is only valid from day 19 or if day of hatching is unknown, when the wing length is 210mm or more then:

Toe span > or = 80 mm is female
Toe span = or < 78 mm is male
From day 19 or winglength 210 mm or more there is no overlap any more in the toe span between male and female and the chicks can be sexed in the nest and the appropriate ring size used.

F rings internal diameter 9mm for male chicks
G rings internal diameter 11mm for female chicks.
 
If there is any doubt also use the weight, birds > or = 700g always female

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Translation of key words in the tables:
Dag= Day
Mannetje= male
Vrouwtje= female
Totaal= total
Gewicht= weight
Klauw zonder nagels (mm)= Claw without nails (mm) = Toe span
Vleugellengte = wing length