The 55% is for clubs outside the Premier League or Football League. For our match v Man Utd its a 50/50 split. (minus VAR/goal line technology costs!)
Taken from this seasons rules:
(b) Subject to Rules 22(k) and (l), net gate receipts shall be divided as follows:
(i) In all matches prior to the Third Round of the Competition Proper the net gate receipts of each match shall be divided equally between the competing Clubs.
(ii) Save for those matches set out in (iii), in all matches played from (and including) the Third Round of the Competition Proper prior to the Semi-Final the net gate receipts of each match shall be divided equally between the competing Clubs.
(iii) In all matches played from (and including) the Third Round of the Competition Proper prior to the Semi-Final, involving Clubs outside The FA Premier League or The English Football League playing against Clubs from The FA Premier League or The English Football League, the net gate receipts of each match shall be divided between the Clubs competing in each match as follows:
55% to Clubs outside The FA Premier League or The English Football League. 45% to FA Premier League or English Football League Clubs.