It is commonly accepted that the easiest way to set up for a f-air combo with Captain Falcon is to use a running down throw into the forward air. More often than not, this actually ends up failing. At what percent is the knockback from the down throw perfect to set up for a forward air, especially hitting with the sweetspot?
The higher the weight the higher the percentage necessary. On average, depending on your timing, 30% is a good starting point. Higher weight characters take around 40 to 50 percent, accounting for the heavier characters, fast falling, etc. Lighter characters only take 25 to 35 percent, again depending on other factors. I experimented and this is accurate and makes the falcon knee hit at least 75% of the time depending on player skill.