How do I add a complex calculation from excel?

There are may ways to accomplish this either by making intermediate hidden fields so one could use the 'field is the ___ of fields:' or the 'simplified field notation' options or you could write a 'custom calculation script'.

Have you looked at the eSeminars on demand [url=https://admin.adobe.acrobat.com/_a200985228/p17266135/]Getting Started with Forms[/url] by Lori Defurio, Thom Parker's tutorial [url=http://www.acrobatusers.com/tutorials/2006/form_calculations]How to do (not so simple) form calculations[/url]? As well as other [url=http://www.acrobatusers.com/learning_center]AUC Learning Center[/url] resources.

You might want to also look at [url=https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference]Mozilla JavaScript 1.5 Core Reference[/url] and Adobe's [url=http://www.adobe.com/devnet/acrobat/javascript.php]JavaScript for Acrobat[/url].

I would follow gkaiseril's advice if you want to know how to create advanced calculations in Acrobat.
If you just need this one, enter the following code as the second field's custom calculation script:

event.value = (((this.getField("SqFt").value / 1000) - 3) * 3.5) + 34;
Note: You might need to adjust the name of the "SqFt" field, depending on how you named it.
Notice that the names are case-sensitive, so "sqft" is NOT the same as "SqFt".