The basic format is IF( CONDITION , TRUE EXPRESSION , FALSE EXPRESSION)
If you want to wrap an existing expression , conditional or otherwise, you need to figure out if the existing expression is the TRUE or FALSE expression for the new condition
if( CONDITION , EXISTING STATEMENT , FALSE EXPRESSION)
if( CONDITION , TRUE EXPRESSION, EXISTING STATEMENT )
A few things you need to watch out for!
1. If(vKickOutcomeMatchTable=0) - You don't want the end bracket here as it's closing your If statement, so get rid of it.
2. Inside your statements you are doing 3 divisions, but I don't think you've wrapped the expression you want calculated first in brackets. If you want to divide "Positive %" field by your expression to calculate the average, you'll want to precede the [Count...] statement with a bracket and end it with a bracket after the second ScenarioMap statement.
3. As a rule of thumb, for every extra if statement you nest, you'll need an extra closing bracket when you reach the end, so if you haven't got the 4 closing brackets if you've got 4 if statements, you know you're in trouble - but use the syntax checker and this should tell you where you're going wrong, if you highlight an end bracket it will show you the bracket it's "closing" and so you should be able to see if you've missed any (not to mention it'll be highlighted red).
It may also be worth replacing your long expression with a variable that stores your expression, as this may help you to see where you're going wrong and reduce the size of the total expression!