Well if you wanted to half the percentage of butterfat then you'd add 100 gallons of skimmed milk, i.e. the amounts would be a 1:1 ratio.

The ratio gives the percentage that you want to keep and the percentage you want to remove.

As you want to keep 16 23rds of the butterfat (3.2/4.6), you have a 16:7 ratio of the amounts that need to be used.

So if 100 gallons is 16/23, then 7/23 is...

(7/23) / (16/23) * 100 = 7/23 * 23/16 * 100 = 7/23 * 2300/16 = 16100/368 = 43.75 gallons of skimmed milk should be added

Does this help your table at all?