I think this just confirms your calculations
Monarch's current excess baggage rates are set out below:
Shorthaul flights - £7.50 (GBP) / €9,00 (EUR) per kg if pre booked online or £10.00 (GBP) / €12,00 (EUR) per kg if booked via our reservations centre or at the airport.
Longhaul flights - £15.00 (GBP) / €19.00 (EUR) or agreed equivalent in local currency per kg
Flights from USA/Dominican Republic/Maldives - $20 (USD) per kg
Note - Longhaul includes Egypt, Kenya, Banjul and Israel