I assume you're talking about a timer controlled in-line or under the carb style stop. I had to kill about .7 or .8 and did it all in the first 2 seconds of the run. Only lost 1 or 2 MPH. You'll have to do it in the middle of the track. If you get too far down track, you won't be able to get the ET into the 10s. Consistency goes away when you run the stop too far down track also. I would recommend a 4 stage timer (drop it once early to kill ET, drop it again at 1000' to kill MPH. Makes it tough to fender race you too) OR some type of stop or bolt on the carb/throttle linkage.