The common high-heat line 220 °C / 425 °F / Gas Mark 7 / about 200 °C fan combines values that are unusually close. Exact conversion makes 220 °C equal 428 °F, only three degrees Fahrenheit above the practical 425 setting. The published Gas Mark 7 row is exactly 220 °C / 425 °F. The fan figure is still approximate: it comes from subtracting 20 °C from the conventional value. Close numbers do not turn the three rules into one rule.
Exact conversion: 220 °C is 428 °F
Using F = C × 9/5 + 32, multiply 220 by 9/5 to get 396 and add 32 to get 428 °F. The recipe shorthand rounds to the common dial value 425 °F. In reverse, 425 °F is about 218.3 °C. This is a small difference, but OvenTemp still identifies which figure is exact and which is practical so users can choose according to the source instructions.
Gas Mark 7 is the fixed 220 °C / 425 °F row
The immutable fixture lists Gas Mark 7 at 220 °C and 425 °F, so this recipe grouping matches the published row directly. OvenTemp returns that complete row when Gas Mark 7 is selected. It does not infer a Gas Mark for 428 °F from the exact calculation, and it does not interpolate between marks. The named setting comes only from the supported lookup table, not from nearest-number matching.
Fan guide: about 200 °C and earlier checking
Subtracting 20 from 220 °C gives the broad guide of 200 °C fan. At high heat, browning and edge cooking can progress quickly, so appliance and recipe guidance matter especially. Confirm whether the original setting is conventional, preheat as directed, and begin checking at the recipe’s earliest doneness cue. OvenTemp provides notation help only; it cannot assess cookware, food thickness, safe internal temperature, or the actual calibration of the oven.