Define a range based on the data in a spreadsheet

I'm afraid I need a little more help.  I get that CNTL-T defines & formats a table.  I've been doing that.  But your link to "structured references" doesn't work for me.  I did find some info in Excel's Help function, but I'm still confused.

Here's the formula I am using:  "=SUMIF($AP$2:$AP54513,"TRUE",$AI$2:$AI54513)" where the values in column AP are either "TRUE" or "FALSE", and if it is "TRUE", I add the value in column AI to the total.  The end value of the table, 54513, is hard coded.  The values in Row 1 are headers.  So, if column AP has the header "GoAdd" and column AI's header is "Sales", how do I use structured references?

I can see how I can start entering a simple formula in a cell and then just click on another cell to get a structured reference to the clicked-upon cell, but how do I use a structured reference to a column, and specifically, how do I replace the SUMIF formula listed above?

Finally, how can I capture the number of rows in a table?

Thank you for your help.