Filling to that fill-plug hole will work fine. However, when the oil get warm it will expand and rise above the hole opening, possibly dripping out a little.
Manuals used to say to refill until the level reached about 1/2" below the filler-plug opening, but to top-up a warm transmission/fluid to the bottom of the opening (until it drips out while filling).
" . . . at operating temperature, lubricant should be level with bottom of filler plug hole."
