Expression for offset postion

I have this expression.

thisComp.layer("Black Solid 3").effect("Particular")("Position XY")

Its is simplely a lens flare (cc lens flare) following the position of a particle (particular).

Particular's emmitter type is box and the actual particle lags behind about 20px.

I do not want to keyframe because of the amount of key points and the speed of movement.

Can i add something to the pick-whipped expression that would allow me to offset the position to be slightly behing TCpaticulars position?  -- (if that makes sense)

BTW, is there a better flare than cc particle in TC?