How to compare Physical Stock vs System Stock and show correct variance?

Hi.

So I work with cases and bottles.

This can mean 6 packs, 24 packs etc...

I want a physical count in Excel with a system count and result to let me know how many bottles I am short/excess etc..

e.g. Phyiscal Count 17 cases and 3 bottles, System count 16 cases and 4 bottles in this instance its a 6 pack so it should tell me I am 5 bottles up but instead a normal calculation will tell me I am 0.99 up..??

Any help greatly appreciated

I should note pack size will be available in the spreadsheet.

Thanks