What's better, scaling CDR or flat CDR? (for runes)

5% flat and 5% scaling, all scaling or all flat gives you a number you cannot reach 40% with, since everything in this game is either 10 or 20% cdr

It depends on the champion you're playing, whether you need the CDR for laning phase because it is an important stat or if you are playing someone who doesn't necessarily need the CDR for laning phase/doesn't spam abilities.

Basically, does your champion need to spam abilities during laning phase or not. Ex. Riven will use flat CDR because she is constantly using abilities and becomes extremely vulnerable without her cooldowns. Ahri is a champion that doesn't need the flat CDR because she isn't constantly spamming her abilities. If she did, she'd run out of mana too fast so you get scaling cdr on her.

I tend to run 10% Flat CDR on my pages, Scaling Runes will give you more value, but you'll have to adjust builds and playstyles accordingly. Mind you, winning a game and then going into another game with the same champion already requires adjustment (in that you need to reset your mentality) but flat cdr is just more reliable to memorize cooldowns with.