Chain Spacing?

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How do teams determine how far apart to place two sprockets to ensure correct chain tension? This year, our robot's biggest issue was the drivetrain, and specifically, its chain.

As an example, I tried using a calculator, and I input 18 teeth for the sizes of both my driven and drive sprocket, and selected 116 links, and the centre-to-centre distance given was 12.25". Upon making a test piece with the same sprocket sizes to test this number, the chain had significantly less links than 116, and was still very loose. How do I figure out how far apart to place the sprockets for proper tension? Do you use calculators? Do you have a rule of thumb? Did you just eyeball it, or get lucky? Please let me know!