Use this table as a guide, temperatures have been rounded for convenience;
Fahrenheit-to-Celsius [Gas Mark]
250 F = 120c [1/2]
275 F = 140c [1]
300 F = 150c [2]
325 F = 160c [3]
350 F = 180c [4]
375 F = 190c [5]
400 F = 200c [6]
425 F = 220c [7]
450 F = 230c [8]
475 F = 240c [9]
500 F = 260c [10]
[Gas Mark] Celsius-to-Fahrenheit
[1/2] 120c = 250 F
[1] 140c = 275 F
[2] 150c = 300 F
[3] 160c = 325 F
[4] 180c = 350 F
[5] 190c = 375 F
[6] 200c = 400 F
[7] 220c = 425 F
[8] 230c = 450 F
[9] 240c = 475 F
[10] 260c = 500F