You should have followed instructions and calculated the numbers of short fall you want in lots of 1000 shares ie $700 (minimum 2000 shares) and Bpayed the combined amount.
ie you prorata was 500 shares than it would have been $350 (70c X 500) plus you wanted buy 3000 more in short fall $2100 (3000 x 70c)
you should BPAYed $2450 and FFX underwriters would have figured out you wanted 500 shares in prorata and 3000 shares in shortfall for you.
Maybe call the company and explain, they might say just bpay the extra