Does anyone know how much STR is Equal to one weapon attack?

Just take the partial derivative with respect to whichever variable you want of the damage range formula: c*(4*STR+DEX)*weapon attack/100 = max damage range

Actually, the OP needs to take the partial derivative with respect to weapon attack and divide it by the partial derivative with respect to STR to get his desired ratio.

∂(Range) / ∂(W.Atk)
∂(Range) / ∂(STR)

= ∂(STR) / ∂(W.Atk)

∂[c/100*(4*STR+DEX)*(W.Atk)] / ∂(W.Atk)
∂[c/100*(4*STR+DEX)*(W.Atk)] / ∂(STR)

c/100*(4*STR+DEX)
c/100*4*(W.Atk)

(4*STR+DEX)
4*WAtk

Oh look. All the constants drop out anyways.