The birth Parkha depends on the age of the person, the age of the mother and on the type of calculation.
There are three calculation methods.
The first called lineair, the second called single zigzag and the third full zigzag.
In all cases take the age of the mother, subtract the age of the person and add 1. ( or take the age of the mother at birth of the person and add 1 )

Lineair
Divide the result by 8. The remainder is used to find the birth parkha.
E.g. Male 61 Mother age 90 gives ((90 - 61 ) + 1 ) : 8 = 3 remainder 6.
Start always at Kham and count anti clockwise the remaining count.
Kham = 1, Khen = 2, Da = 3, Khön = 4, Li = 5, Zön = 6 which is the birth parkha.

Single zigzag
The single zigzag method is more complex as there is a diagonal jump at a count of 10
E.g. Male 61 Mother age 90 gives ((90 - 61 ) + 1 ) = 30
Start at Kham = 1, Khen = 2, Da = 3, Khön = 4, Li = 5, Zön = 6, Tsin = 7, Khin = 8, Kham = 9, Khen = 10.
Now do a diagonal jump to Zön = 11, Tsin = 12, Khin = 13, Kham = 14, Khen = 15, Da = 16, Khön = 17, Li = 18, Zön = 19, Tsin = 20, Khin = 21, Kham = 22, Khen = 23, Da = 24, Khön = 25, Li = 26, Zön = 27, Tsin = 28, Khin = 29, Kham = 30..

Full zigzag
The full zigzag method is the most complex as there are jumps at a count of 10 and than jumps at every count of 11.
E.g. Male 61 Mother age 90 gives ((90 - 61 ) + 1 ) = 30 count
Start at Kham = 1, Khen = 2, Da = 3, Khön = 4, Li = 5, Zön = 6, Tsin = 7, Khin = 8, Kham = 9, Khen = 10. Now do a diagonal jump to Zön = 11, Tsin = 12, Khin = 13, Kham = 14, Khen = 15, Da = 16, Khön = 17, Li = 18, Zön = 19, Tsin = 20, Khin = 21
Now do a diagonal jump to Khön = 22, Li = 23, Zön = 24, Tsin = 25, Khin = 26, Kham = 27, Khen = 28, Da = 29, Khön = 30.
Algebraic: (count + ((count : 11) * 3)) : 8 -> (30 + ((30 : 11 ) * 3 )) : 8 -> (30 + ( 2 * 3 )) : 8 -> 36 : 8 = 4 and remainder = 4 use remainder start at Kham = 1, Khen = 2, Da = 3 and Khön = 4