Giant reign 2 2018 sag %?

Sag is just a quick way to get close to the correct spring rate (air pressure assuming it's an air shock) and is usually around 25-30% however that's just a guide and you need to play around and see what feels best for you. It's worth taking a shock pump with you for your first few rides and have a play with slightly higher and lower pressures.

as a big bike I'd run a bit more sag than a normal bike (say 35%), if you find yourself bottoming out regularly then stiffen it up a touch. The old advice used to be one clunk per ride shows you're using all your travel

Thank you both