You are right, the fields you are interested in are "hidden"

Add /A to your GridView shortcut and you will see the "hidden" fields. As a matter of fact I always put the /A on my shortcuts.

Here are the Fields you will see:

Inv Total - PTD
CN Total - PTD
DN Total - PTD
Disc Total - PTD
Int Total - PTD
Inv Total - YTD
Payment Total - YTD
CN Total - YTD
DN Total - YTD
Disc Total - YTD
Int Total - YTD
Inv Total - LY
Payment Total - LY
CN Total - LY
DN Total - LY
Disc Total - LY
Int Total - LY

Put these fields on a View and determine what ones you need to calculate your numbers.

In GridView the equivalent of:
if d>=bofp1 then a
is:
if (d>=bofp1,a,0)

"d" might be {Order Date} , "bofp1" might be the result of a Querydate and "a" could be {Total Order Value}

so define a calculated column bofp1 to be QUERYDATE("Enter Start Date", Today())

Sales Amount calculated column would be

if ({Order Date} >= bofp1,{Total Order Value},0)

However, I would simply define a filter to get the OE Header records that were of interest and then you could use the {Total Order Value} without any other calculations.