@kloudie1 is right, have to look at the interactions, but not just in B&R rigs. Fairly universal that the headstay should be set to a length. True for all basic/common rigs (masthead, frac, B&R, in-line spreaders, swept-back spreaders). You adjust the forestay length to change the rake, which changes the balance of the boat. Longer forestay = more rake = more weather helm.
Forestay tension changes the forestay sag, but this is controlled by cap-shrouds on a swept-spreader; back-stay on a masthead online spreader; or running backs on a frac rig with inline spreaders.
@Pat cool skip my guess is you want to change the forestay sag, is that correct? Need to tighten the cap shrouds... might have to be pretty dang tight to get the upwind forestay tension you want... which is a compromise for downwind performance. Alternatively, you could add running backs as well, if you want more adjustability, and the potential to optimize both upwind and downwind sailing.