I use my runners to get the correct luff sag.
The headstay and forestay 'always' variably 'share' the single reaction of the backstay, all depending on wind conditions and how much each foresail is 'pulling'. What always happens is that the larger headsail shares its load 'into' the forestay, the forestay tightens, and the headstay then 'unloads' and sags off to leeward. A leeward sagging headstay is a surefire method to get a boat to start skidding off to the lee.
FWIW ... in 'cutter speak' the stay directly in front of the mast is the forestay, the stay furthest in front of the mast is the headstay - just to keep the terminology straight for this discussion.

Applying runner tension to bring the headstay sag back to normal is the standard way to do this, as long as you dont go much above 30% tension in the wire which will accelerate fatigue (failure). Next time also try 'unloading' the tension in the forestay and more of the reactance backstay tension will automatically go to the headstay ... and then you wont have to overload the rig so much with a running backstay.

Here's how make a cutter rig point like a banshee: http://www.ftp.tognews.com/GoogleFiles/Matching%20Luff%20Hollow.pdf and use any means to get the 'interplay' tensions of the headstay/forestay combo right, either by runner tension or slacking off the forestay, etc.