Set a Column in Pivot Table as Percentage of Another

I believe what you want is a calculated field.

From the pivot table "Options" menu for Office 2010 or before or the "Analyze" menu for office 2013 or after, select "Fields, Items and Sets" -> "Calculated Field."

In the Name box, name it whatever you want ("% of SLA Met").

In the Formula Box, you can click on the fields to have them brought up, and it will end up looking something like this:

='Responded SLA Met'/'Ticket Nbr'

That said, you can't really do a "sum / count" -- you can, but it won't do what you want. So, you will probably have to add a column with all 1's in it (call it ticket count) and then use the sum of that as the denominator:

='Responded SLA Met'/'Ticket Count'