I'll just ask a question first. How will you calculate the volume of beer racked to the bottling bucket? Factory markings on buckets are notorious for being wrong. The only way to know the real volume is to mark the bucket yourself with measured volumes of water. The fifth gallon could be measured in quarts.
Racking the beer first to find the true volume will require stirring the beer to mix the priming sugar well. A larger volume of water with the dissolved sugar will give better mixing results. Stirring too hard also risks oxidation.
At 68°F for 2.4 volumes of CO2 in 5 gallons of beer will take 118 grams of corn sugar. 118 grams of corn sugar in 4.5 gallons of beer will result in 2.6 volumes of CO2. Not too much of a difference.
Do you weigh the amount of priming sugar used or use volume measurements? Weighing is more accurate.
Are you sure your first over carbonated beer was done fermenting before it was bottled. I'm working with this problem right now. Even though the specific gravity held steady for 6 days the SG dropped 7 points in the bottle over the next month. Not caused by infection either.