FIFO worksheet decimal failure

[youracct]

I have the following worksheet which works pretty well with the VBA code, except that when I put decimals in column A it throws off the FIFO inventory for some reason. Unfortunately we need to use decimals in column a. I have tried creating helper columns that convert the tons to pounds, but it causes the spreadsheet to lag something fierce. Any ideas on why the decimals cause this to fail? I have pasted the vba code I am using under the worksheet below.

Thanks!

purchased tons	price/ton	tons used	FIFO Ending Value
19375	7.8565	3375
14211	.3518	3234	105296.32
15605	.6408	42572	6.41
120000	2.9167	11910	291.67

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Option Explicit


Function FIFO(PurchaseUnits As Range, UnitCost As Range, UnitsSold As Range, Optional blnAscending As Boolean = False) As Double
Dim Counter As Long, UnitsAccountedFor As Long
Dim varPurchased, varCost
FIFO = 0
UnitsAccountedFor = Application.Sum(UnitsSold)
varPurchased = PurchaseUnits.Value
varCost = UnitCost.Value
If blnAscending Then
Counter = LBound(varPurchased, 1)
Do Until UnitsAccountedFor = 0
If varPurchased(Counter, 1) = 0 Then
Counter = Counter + 1
Else
varPurchased(Counter, 1) = varPurchased(Counter, 1) - 1
UnitsAccountedFor = UnitsAccountedFor - 1
End If
Loop
Else
Counter = UBound(varPurchased, 1)
Do Until UnitsAccountedFor = 0
If varPurchased(Counter, 1) = 0 Then
Counter = Counter - 1
Else
varPurchased(Counter, 1) = varPurchased(Counter, 1) - 1
UnitsAccountedFor = UnitsAccountedFor - 1
End If
Loop
End If
FIFO = Application.SumProduct(varPurchased, varCost)
End Function