Resource Cost

Hi,

I have one Resource "Furnace" having Per Hour Rate of Rs.10 and Overhead per Hour rate of Rs.100 

Assembly A, B & C each requires 40 Hrs. of Furnace. One furnace can process 100 Assembly numbers at any time.

This resource is included with usage of 40 and basis as Lot in Routing of Assembly A, B & C.

Now if I process 10 quantities of Assembly A Furnace usage cost will be (10*40)+(40*100)

If I process 50 Qty Assembly A, 30 Qty of Assembly B & 20 Qty of Assembly C in Furnace at any time.

Then my actual usage cost is 10*40+100*40=4400

But system is giving cost as 4400+4400+4400=13200

How I can map this requirement of getting correct cost for this resource?