There isn't a "standard dot". Some instruments have 5 dots per side (full deflection), while others might have only 2 dots or 3 dots for the same deflection. At the extremes I've seen older instruments with no dots at all and one with 10 dots per side.
Assuming a 3 degree GP, on a 5-dot display one dot is approx. 0.15 degrees. On a 2-dot display, one dot is approx. 0.35 degrees.