I read through winwoods instructions (first one's that came up on google) thinking i'd get a straight answer.

http://www.winwoodbike.com/pdf/Winwood_fork_inst.pdf

"Using a 5mm hex key, turn the expander bolt clockwise until the expander ring tightens firmly against the inner diameter of the steerer tube. DO
NOT OVERTIGHTEN."

Just use a Tri-Allen wrench and turn pretty hard, don't put all you've got into it. If you end up not setting it tight enough the only bad thing that could happen is that it'll slip while setting headset preload. And if it does slip, you'll know and you can just tighten the plug slightly more. So if you have to err on one side, err on the side of 'not tight enough'.