I don't think anyone would expect you to know this but in foetal Hb [HbF] the amino acid at position 136 in the gamma chain is replaced by alanine or glycine [adult haemoglobin [HbA] has 2 alpha chains and 2 beta chains, WHEREAS in HbF there are 2 alpha chains and 2 gamma chains].
The alpha chain is 141 a.a.-s long, while both the beta and gamma chains are 146 a.a. long.
So, if we consider this one substitution [above] there would be a difference of 2 a.a.-s out of [141 X 2] + [146 X 2], so 2 out of 574, which is approx. 0.4 % - I have no idea how the examiner calculated the massive %age. - SORRY!
Can you check for OP please Jpw1097 - thanks!