What does multiplying the value by the use mean? That sounds like "I will use this X times, and each use is worth Y, so the result is X*Y." If that is what you are saying, that doesn't seem to assess the overall value very well because it is possible (and likely) that not all uses are worth the same; there is no fixed Y.
Of course, you could always figure out the total value first, then divide by X to get a per-use value. This value would work mathematically, but doing that doesn't get you anything, since you're really interested in the total value anyway.
We had a 3-point system of value for each part of the house "1=low, 2=moderate, 3=high" and the same for use, with use averaged year-round.
So for the backyard example: the backyard for house 170 was high value, 3, and high use for half of the year, so 1.5. Its total value was 4.5. The backyard for house 450 was moderate value, 2, and high use for half of the year, so 1.5. Its total value was 3.
My wife and I have been going to Ohio Rationality Dojo for a few months now, started by Raelifin, who has substantial expertise in probabilistic thinking and Bayesian reasoning, and I wanted to share about how the dojo helped us make a rational decision about house shopping. We were comparing two houses. We had an intuitive favorite house (170 on the image) but decided to compare it to our second favorite (450) by actually shutting up and multiplying, based on exercises we did as part of the dojo.
What we did was compare mathematically each part of the house by comparing the value of that part of the house multiplied by the use of that part of the house, and had separate values for the two of us (A for my wife, Agnes Vishnevkin, and G for me, Gleb Tsipursky, on the image). By comparing it mathematically, 450 came out way ahead. Hard to update our beliefs, but we did it, and are now orienting toward that one as our primary choice. Rationality for the win!
Here is the image of our back-of-the-napkin calculations.