s = """
A B C
  1    0.1          300
  2    0.2          400 (11 - this text is part of C in row 2 but needs to be ignored / removed)
  3    0.9          600"""

print(my_dict)
{'1': {'B': '0.1', 'C': '300'}, '2': {'B': '0.2', 'C': '400'}, '3': {'B': '0.9', 'C': '600'}}

s = """
A B C
  1    0.1          300
  2    0.2          400 (11 - this text is part of C in row 2 but needs to be ignored / removed)
  3    0.9          600"""

#Get the columns and assign them to a variable.
columns = s.lstrip().splitlines()[0] #Print the first line of the string

dct = {}

rows = s.lstrip().splitlines()

for data in rows[1:]:
    row = data.split()
    dct[row[0]] = dict(zip(columns[1:], row[1:]))

print(dct)

{'1': {' ': '0.1', 'B': '300'}, '2': {' ': '(11', 'B': '400', 'C': '-'}, '3': {' ': '0.9', 'B': '600'}}