Celsius and Fahrenheit: a change is not a reading
Convert temperature readings with the offset, but convert a temperature difference using only the scale factor.
Method
A temperature reading converts with °F = °C × 1.8 + 32. A change converts with Δ°F = Δ°C × 1.8. The 32-degree offset belongs to the location of zero on the scale; it disappears when subtracting two readings. Write down whether your input is a measured temperature or the difference between measurements before choosing a formula.
Worked example
A reading of 20°C equals 68°F, and 30°C equals 86°F. The increase is 10 Celsius degrees or 18 Fahrenheit degrees: 86 − 68 = 18. Applying the reading formula to the increase would give 50°F, which is a temperature reading rather than the size of this increase. A decrease of 10 Celsius degrees corresponds to a decrease of 18 Fahrenheit degrees.
What to check
Use the Temperature calculator to convert each actual reading and subtract the results if you need a change. A difference of 10°C also has the same numerical size as 10 K, whereas the absolute reading 10°C is 283.15 K. Avoid describing the rise from 20°C to 30°C as a physical 50% temperature increase: ratios on offset scales depend on the chosen zero. Keep the measurement precision rather than adding unsupported decimal places.