Want to lowering my coupe a little, high as a truck now

But it's stiff engouh as it is, so just think i'm gonna lowering it with springs..

But how much to get it loock good?
Don't want a big difference in the front against the back of the car or otherwise..
30 in the front and 40 in the back any good?

What's springs are good are to use at standard shocks?
Have astra mk 4 turbo coupe..
THANKS!!!