How much should I lower my celica?

I bought my 2000 Celica gts off a guy on craiglist. The car was lowered when I bought it and I dont like the ride quality. The reason why is because the dude cut the springs! I have to go extremely slow on speed bumps because then ill bottom out. My ride is also bouncy. I was thinking on buying 1.4 lowering springs to have a stiffer ride and have the low look. The thing is I dont know if bottoming out will be a problem when going over speed bumps and speed humps.I know the celica looks pretty high stock which is why I want to lower it but how many inches should i drop it without having speed bump problems?