It depends on where your probe is. If you have it where you should, ~3" from the head, then you should stop whatever you are doing at about 870 C.
You have to project ahead what the temp is by a few seconds. Expect the temps to be whatever the gauge shows in a few seconds at the current rate of increase.
You are technically safe by most opinions up to 900, but that's actual, not indicated. Forged internals (which you don't have) can withstand a little more, but it's not worth tuning to that edge on a street car.
I usually keep my warning light at 870 or so. If I'm tuning, I lower it to make sure I know when I'm even close in case I'm too lean and it's rising fast. If I am pulling hard through 2nd on up and I hit 870 slowly while creeping up in 4th or 5th, I'm not usually too concerned until I get to 880~890.