100 percent view actually shows 50 percent

Hi

I've been using C1 Pro 9 on an old Mac. Now I've got a new 4k iMac and it runs High Sierra. All working well, except when I go to 100 percent view it actually shows 50%. I don't know if this has happened because of the upgrade to High Sierrra or the upgrade to a 4k screen.

Can I force it to go to a true pixel view easily, or will I have to drag the slider to 200% every time?

Thanks

Paul