there are many scale rules available with multiple scales, usually N, HO, S, O, inches and mm. I reccomend one of those so there are no problems with converting from scale to scale.
Actually, there is a sound rationale for using 1/8 in per foot for structures on an HO railroad:
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Many buildings are simply too big to fit when built to exact scale.
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The “art” of model railroading includes selective compression. This includes reducing the number of windows, bays, etc., but also reducing the scale.
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Buildings located toward the rear of the layout are often intentionally built to a smaller scale to make the railroad look bigger (deeper) via “forced perspective.”
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Some find 1/8 inch easier to work with than 3.5 mm, 'though with scale rulers and calculators this isn’t a major consideration. But if you need a 12 by 12 timber, the widely available 1/8 inch square stripwood is right on!
Have fun.
John Frank