Whats the ideal offset for 11.5x19 (305/30/19)rear and 8.5x19(235/35/19) fronts , dont want to roll the arches but want them filled as much as possible. Car is lowered around 2" on JIC cross coilovers and running GT2 spec alignments.

 

Well....for the fronts...below is example of 42mm.
Not sure how low you are (#s @standard suspension measure points) but when you get down there, your suspension firmness & alignment play rolls in whether you need fender prep or not.....but assume you know this since you've already mentioned as much.

Rears....some here run 11s with ballbark 30mm offset (with 315s)....so simple translation of that to 11.5s .....you'd be around 36.

 

Thanks alot
This is how the car looks justnow(front has been dropped slightly since) with a 10mm spacer on the rear, was thinking of moving that to the front and fitting a 18-22mm on the rear.

 

Just checked my wheels and there ET50 front and rear so would i get away with a 10mm spacer front and 22mm rear.

 

Probably OK.
If you lower the back any more (using OE suspension & camber) you get to where you may need fender prep.....tighter suspension and-or more aggressive camber may avoid that.

 

This car looks sick! I'm going to have to paint my black plastic and get a C4S front lip!