Half Yearly P&L prediction

Half yearly predictions with last year's actuals for first six months in brackets: $Amil

Revenue from mining operations 224(150)
Cost of sales 195(121)
GROSS PROFIT from mining operations 29(29)
Provision for loss in Property JV -2(0)
Administrative expenses -3(-3)
Exploration & evaluation costs -1(-1)
Net Finance income 2(6)
PROFIT before tax 25(30)
Income tax expense 6(9)
PROFIT for half year 19(21)

Calculation of Cost of sales figure is the very difficult one as it comprises lots of figures, some of which require a high degree of outright guessing. But my dissection of these figures is as follows(with comparatives in brackets):

Mining & Production costs 123(119)
Changes in inventories 26(-27)
Sub-total: Mining & Production costs(net) 149(92)
Freight costs 5(6)
Govt royalties 7(3)
Deprec & amort - Plant 18(6)
Mine properties & development:
- Amortisation 4(3)
Deferred stripping:
-Amounts capitalised -6(-4)
-Amortisation 18(16)
TOTAL Cost of sales 195(121)

So on a 50% increase in Mining revenue, I've got what is essentially a flat profit - sorry but that's the way I read it.
It does mainly revolve around a probable drop in the volume and value of ore stocks on hand between Dec19 and Jun20.