How much fresh potato to substitute for instant potato flakes when baking?
Instant potato flakes are effectively dehydrated mashed potatoes. 

In general for potato flakes you mix 3:4 volume flakes to water. They increase in volume by about two and a bit times, and weight about six times

For potato powder you mix 1:5 volume powder to water. They increase in volume by about three times, and weight about six times.

So roughly 3.75 ounces of water are required to rehydrate .75 ounces of potato flakes which gives 4.5 ounces of mash, or just less than 2/3 cup of mash

Remember to remove the equivalent water (3.75 ounces) from the recipe

This all varies slightly depending on the supplier of flakes and what variety of potato they used