How much of tankadins aggro comes from white damage?

As topic name, Im wondering on how much the weapon we're using (-spelldamage & other effects) affects our aggro? As I see most spelldamage weapons are quite low dps wise.

And how much does lets say 1 point of spelldamage affect our aggro compared to lets say 1 point of normal damage in the weapon?

Also Im wondering that at which amount of spelldamage should i switch from using a normal weapon into using a spelldamage weapon?
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Aggro from white damage is normally between 5 and 10%.

To add to Lore's post, since most spell dmg weapons have 40-42 white dmg, and most tanking weapons have around 87-92 white damage, the break even point is when your spell dmg weapon has around 80 spell dmg on it as that will give 50 spell TPS roughly
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In a heroic I did recently, melee damage accounted for 12.5% of my total threat.

I was AoE-tanking a significant portion of that run, and as a result a quarter of my melee attacks were because of reckoning procs...suggesting that without Reckoning about 9-10% of my threat would have been from melee.

This was using a Continuum Blade (41.4DPS) with 224 unbuffed spell damage.
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Scarlet Crusade - 0/46/15 - Uncrushable at 15.2k