Inventory Forward Coverage

[J0HNDOE]

Im trying to do a forward coverage calc in terms of months, but not to exceed 12. The formula I have is =IF(E3<=0,0,IF(SUMPRODUCT(--(E3>=SUBTOTAL(9,OFFSET(B3,0,0,ROW(INDIRECT("1:17"))))))+LOOKUP(0,SUBTOTAL(9,OFFSET(B3,0,0,ROW(INDIRECT("1:17"))))-E3-B3:B19,(B3:B19-SUBTOTAL(9,OFFSET(B3,0,0,ROW(INDIRECT("1:17"))))+E3)/B3:B19)>12,12,SUMPRODUCT(--(E3>=SUBTOTAL(9,OFFSET(B3,0,0,ROW(INDIRECT("1:17"))))))+LOOKUP(0,SUBTOTAL(9,OFFSET(B3,0,0,ROW(INDIRECT("1:17"))))-E3-B3:B19,(B3:B19-SUBTOTAL(9,OFFSET(B3,0,0,ROW(INDIRECT("1:17"))))+E3)/B3:B19))) with the attached layout. The 2 issues I run into are #1 if my on hand is greater than expected demand it errors out rather than displaying 12 and #2 if my on hand is equal to one month of demand and the following months demands are 0 then it says those months are covered even though I would have 0 on hand. Any help appreciated.