If you are only going to store the length of the wall and how many studs by foot, why not only one table?
Code:
walls: wall_id, title or description, length_in_feet, studs_by_foot, allowance
if:
length_in_feet = 24
studs_by_foot = 1.33
allowance = 0.03
Calculation:
units = length_in_feet * (1/studs_by_foot) * (1 + allowance)
24 * (1/1.33) * (1 + 0.03) = 18.58 units
or
Code:
walls: wall_id, title or description, length_in_feet, stud_separation_in_feet, allowance
length_in_feet = 24
stud_separation = 0.75
allowance = 0.03
Calculation:
units = length_in_feet * (stud_separation_in_feet) * (1 + allowance)
24 * 0.75 * (1 + 0.03) = 18.54 units
Look at the calculations, one is the inverse of the other. That can be solved easily with a field where you store whether the values will be multiplied or divided to get the right ratio.
Still, it is unclear what kind of thing you want to do here. You want to find out how many studs there are in one wall? That's fair, will you later need horizontal elements too?, are they even the same kind of material? how about the frame? how about the windows? how about the places that will require additional support because there's going to be a TV? how about doors? And what are you going to do once you know how many studs? Do you need to have a number of studs? what are you gonna do when the walls are taller? or shorter? it's not the same stud length. You will probably need to account for height here, and type of element. Then you would need to remember they are sold in standard lengths, how are we accounting for that here?
Perhaps, if you're going to store several types of measurements, you'd need a more universal approach with a table for the project, another for the kind of measurement (concept), another for the type of unit, another for the resource necessary and another for the concept detail. You would then have:
Code:
projects: project_id, title, created_at, updated_at
concepts: concept_id, description, unit, quantity, project_id, created_at, updated_at
units: unit_id, title, abbreviation, created_at, updated_at
resources: resource_id, description, unit, created_at, updated_at
concept_details: concept_detail_id, resource_id, quantity, price is optional, concept_id, created_at, updated_at
And provide the tools for each type of concept.