Everyone who’s done slate has done this at least once. You slate take 3 and while the scene is being shot you erase your 3 and write in a 4. Then the scene is over, you check the slate and forget that you’ve already incremented the take so you erase it again and write 5.
Try this instead: Slate take number 3 then automatically draw a diagonal line through the 3 as the scene is shot. Now at any time before the director calls for a new shot you know for certain that take 3 was slated and the scene was begun.
If the director calls for another take you can erase the crossed-out 3 and write in a fresh new 4. If the director calls a cut-print-moving on then you increment the shot letter and erase the crossed-out 3 and write a fresh new 1.