You will need 5 buttons. Rowan used #A0478 from Groves & Banks

Finished measurements:
Back width: 22(24, 26, 28, 30)“ 56(61, 66, 71, 76) cn
Finished length: Approx. 25(25.5, 26.5, 27, 28)” 63(65, 67, 69, 71) cm
Sleeve length to underarm: 17.5” for all sizes 44(44, 45, 45, 45) cm