-20 isn't effective enough. Resistance should be just as powerful as Weakness so if you can easily lose to weakness, they you should also be able to lose to resistance so -60 for it is big enough to change matchups completely.
Which is what we don't want, or at least I don't. The whole Weakness/Resistance concept worked for the original Game Boy games, when we didn't have as many Pokémon and the all-or-nothing Type "packages" fleshed out Pokémon. Now? I'm pretty sick of it. ^^' There are way too many Types for the janken style of gameplay to feel remotely balanced. Honestly, I'm not sure if ever really worked properly.
I'd abandon it entirely, but I doubt that is an option. Resistance isn't as effective as Weakness because offense includes an aspect of defense (KO them before they can KO you XP). I'd adjust the numbers for Resistance but I have the feeling that it will be best to keep it perfectly symmetrical. So +20/-20 still makes a difference in edge cases, but not much else. Sounds like a decent compromise to me.