I plant a separate patch for use during the Summer and dig the whole plant when required. The main plant is left undisturbed until die off for storage.
I correlation between seed planted and production is meaningless, since if the potato is cut with one eye it can skew the results. When I first started planting potatoes I could find nothing about what to expect per plant. Now I have reasonably good records. I consider anything over four pounds to be acceptable. The range is 4 to 8 pounds.
Mind you quantity by weight is not the only criteria. Overall quality is determined by evaluating number of potatoes, size of potatoes, depth of eyes, no knobs, no scabs, appearance of cut texture, Some of the determination is subjective to some degree.
Here is one of my efforts, which I use for comparison.
http://www.durgan.org/URL/?BKWAI 11 September 2010 Yukon Gold Test Box Potatoes
Yukon Gold Potatoes were harvested today. A total weight of 23.5 pounds was harvested from the 4 by 4 foot test area. The quality is excellent. Another plant could probably be placed in the center of the area without crowding. The average weight per plant was 5.9 pounds. From my experience anything over 4 pounds is acceptable.