Just get him as high as you can. He'll still be good at sig 20 but you ideally want him at sig 200
Void's sig ability has a very steep curve so the more sig stones you can put into him the better, ideally best at sig 200 but anything above 150 is must in my opinion This is bad advice. Void needs sig 1. He benefits from more. But NEEDS sig 1. Id recommend dumping them into him, but if u have CAIW needing them first, use them on CAIW.
Void's sig ability has a very steep curve so the more sig stones you can put into him the better, ideally best at sig 200 but anything above 150 is must in my opinion