i dont understand why, yes SF has much total potency higer than ruin , but if isnt affected by RS is still better cast before RS and u can do Ruin1 and if u have mana to waste a Ruin2 before RS ends,then recast SF when expires
in this case RS has same potency in every cases u use so as u wrote 350 but ruin1 and 2 has 20% bonus that u loose if RS expires
if u are talking about "its more handly" for calculate timing maybe it can be true ,i never tryed it,but speaking in term of dps is far better cast out of RS and gain those 100 dmg of the two boosted ruins,isnt so much so can be forgiven but is higher anyway