Deepblue12

I'm using Cura 5.2.1 with a Creality Ender 3 Neo printer. I've replaced the original bed with a magnetic PEI plate that does not require clips. The bed size is stated as 235 x 235 and i'm trying to print a circular disc which is 220mm wide total. Cura has a thin grey area at the front of the plate which does not allow me to slice unless i reduce the disc size down to around 213mm. I've found a link that suggests editing the json file and removing the restrictions on printable area but when i try and save the edited file it gives me permission issues which i cannot seem to resolve. Is there a simple fix for this...any help appreciated