The temp difference is about 250-300 degrees. I too have a POST turbo EGT and this is the rule I use. Under heatsoak conditions, like when your stressing the turbo for a long time, the EGT's can actually be higher on the POST but will go back after the turbo caseing cools. I would HIGHLY recommend getting another probe and having it installed PRE turbo. I'll be getting an all new EGT gauge so I can monitor PRE, and POST EGTs simultaniously and be able to see when I'm getting heatsoak. (If you go this route, be sure to lable the gauges so you or anyone else that might drive it won't get confused.)