Sorry, I only check this every few days also.
If you have a zero roll car, besides the static negative camber, you have a droop stop. The sudden oversteer could have been the car coming up on the droop stop. when you jack the car up in the air, the droop stop should hold the camber to negative about -1 to -2 degrees. The amount is not as critical as coming up against the stop is.
By going more negative, you reduce the chance of getting into the stop. On an autocross, you make a lot of quick transitions which could cause the car to jack. Not sure what tires you are running but at some point you go too negative and then you start having oversteer again. But if you are around -5 degrees total and -2 total in droop, then it should be runnable. You can also look for a softer stop rubber.
Your method of checking toe is okay, I use a similar method myself. The tried and true method is to jack the car up in the air so the tire is just off the ground and scribe a line around the middle of the tire. This video https://www.youtube.com/watch?v=0ziovI1651o
shows how he runs a loop of tape around the tire and marks it with a pen (not shown). I will not vouch for the rest of the video, but you can do an alignment with string (I use heavy fishing line) If you want a professional tire scriber it looks like this http://www.longacreracing.com/products. ... ire+Scribe
but I have used a sharpened screwdriver and a block of wood to hold the angle.
Here is the scriber in action: https://robrobinette.com/images/S2000/A ... Scribe.jpg
Using your method, I would just make two marks on your straight edges, matching the diameter of your tires. the difference is your toe. If you measure 58" and 57 15/16" - then you have 1/16 total toe. You have to use some common sense to decide if it is toe in or toe out depending which measurement is where. If my car was not apart I would do a video - I will have to add this to my list. I don't know how you are measuring your .8 degrees so I cannot tell if it is a lot or not. More toe in in the rear gives stability, but on an autocross you need just enough to make the car comfortable to the driver.
Good luck with your next event and let us know how you do.