Lookup table with exceptions?

I'm currently using a lookup table to determine programs to run on a machine, however I have 3 exceptions to this and can not get my code to work.

Currently the code looks at column D for the type, then column F for the thickness of material. It then calculates the etch time required using the rate (all shown as 1 currently) on the cal_sheet and then rounds this up to a whole number. This is then compared to a second table where the program details are listed against etch times.

It will make sense when you look at the attached workbook and read the code, honest!

Currently using the following code:

=IF(F5="","",LOOKUP(ROUNDUP((F5/(LOOKUP(D5,'Calc Sheet'!F:F,'Calc Sheet'!H:H))),0),'Calc Sheet'!$K$1:$K$44,'Calc Sheet'!$J$1:$J$44))

This works for everything except my exceptions.

My proposed new version is:

=IF(F6="","",(if(D6="A","A program",(if(and(D6="B",F6<16,"B Thin",(if(and(D6="B",F6>16,"B Thick",LOOKUP(ROUNDUP((F6/(LOOKUP(D6,'Calc Sheet'!F:F,'Calc Sheet'!H:H))),0),'Calc Sheet'!$K$1:$K$44,'Calc Sheet'!$J$1:$J$44))))))))))

But I keep getting an error, highlighting the second LOOKUP.

Can anyone help please, it probably a simple problem, I just cant see it.

Thanks,
Gavin.