FIFO Calculation Help using Function

Hello

I am hoping this is an esay thing to fix but I have found a FIFO function to use in Excel and it doesnt seem to work. My VBA is basic and I cant step through the code as it is a function. All I basically want to calculate is the FIFO value of the sale quantity. As you will see in the attached its calculating some weird values.

For the Sale on the 1st Nov I would expect the following:

323 x 640 = 206,720

For the Sale on the 2nd which goes across two inventory values I would expect the following:

3,409 x 640 = 2,181,760
392 x 668 = 261,856
= 2,443,616 as sale USD value FIFO and then this would come off the closing value.

Thanks for your help.

Thanks for the info, allows me to debug but doesnt get to the bottom of my issue, still not working correctly. Please help.