You can display a value from any field in the data editor in the tooltip using this format:

<d ref="Behavior,LethalFury,Duration[0]"/>

<d ref="Behavior,Lethalfury,Modification[0].DeathResponse[0].Cost[0].Cooldown[0].TimeUse[0]"/>

<d ref="Behavior,LethalFury,Modification[0].AttackSpeedMultiplier[0]" precision="3"/>

It works for effects also, as such:

<d ref="Effect,TimeOrbSearch,AreaArray[0].Radius[0]"/>

<d ref="Effect,RepeatedBlows,Chance[0]" precision="3"/>

From what I understand, it's simply a data editor reference, pointing at a field value? Am i rite?

The first problem I'm encountering is how to display a derived value - ie. two field values multiplied together.

For example this:

Effect,TimeOrbPersistent,PeriodicPeriodArray[0] * Effect,TimeOrbPersistent,PeriodCount[0]

will translate to

0.0012 * 6000

Resulting in 7.2

But I tested that calculation out in the 'Formula Editor', and as such it removes the <d ref= "" /> tags. Therefore, simply pasting that into the tooltip field will show "Effect,TimeOrbPersistent,PeriodicPeriodArray[0] * Effect,TimeOrbPersistent,PeriodCount[0]" as plain text.

Question 1: What is the xml / starcraft 2 convention for converting the 'math' back into 'text'? Ie. How can I display the outcome of a mathematical function in a tooltip?

Question 2: What is the 'array' value at the end of every example posted above? eg: "ObjectType,ObjectName,FieldValue[array value]" ? What specifically does that point to? Are abilities/effects/etc able to be given 'levels'? Or does this refer to 'upgrades'? Or are all effects/abilities duplicated per player (to allow for upgrades etc?)

None.