How to Customise PermGen?

[Dban1]

 Hi, I've been trying ways to increase PermGen for my private modpack with close to 400 mods, but the thing is that I heard too much permgen is bad and too little will cause severe fps drops after 10 minutes of playing (which is what I'm experiencing now). So is there a way to customise the permgen and permgenmax to 256 and 512 from the launcher? 

 

This is what I typed in the Java Arguments box exactly

-XX:PermGen=256M -XX:PermGenMax=512M

No colons, case sensitive, space sensitive.

I'm not sure if this is working, but before I move on to another fix, is this the right way to edit permgen?

Edited November 23, 2015 by Dban1

[E4RU]

Try this one? -XX:MaxPermSize=500M

Edited November 25, 2015 by E4RU