Adjusting furler for headstay sag

[Sailormon6]

My friend's 28' boat has an old Hood Seafurl jib furler, and it has a little too much headstay sag. Can someone tell me if the amount of headstay tension on that furler is adjusted by the halliard, or is there some other way of adjusting it. (I think I could figure it out by a little experimentation, but I hate to experiment with a friend's boat when I'm not sure what I'm doing.) Thanks for your help!

[Stillraining]

Not familiar with that furler..but on most boats once your forestay is made to the correct length back stay tension controls it and mast bend.