Having trouble putting a formala together.

I have the following formula which works:

Senario : $d$d = either weight or unit or unknown (of an ingredient)
e8= is a quantity number ( either in grams or units, of an ingredient. eg 100 or 1)
Raw data is in nutrition!$b$6:$g$70

The following formula works so far:

f8 ( which is the no. of calories)=if($d$d="weight",e8/100,e8)*vlookup(c8,nutrition!$b$6:$g$70,3,false)

But I also want to add to it that

If e8(quantity)=0 or blank or invalid then "qty?" should appear in the f8 cell

And not sure what to do to avoid an error in f8 if $d$d = unknown??

Am completely stuck here, need help asap.

lozzam