I prefer to use HM x 2.1 which is in line with times two plus 8 to 10 minutes for a three hour marathon runner. But I find it is more accurate for how you're running at the time, especially if sub three would be a bit of struggle given current fitness

My best HM was set well before I stepped up to the marathon, but the last half I did before my PB of 2-38-40 was 73-52 which is a relatively poor + 11 minutes but I was aiming at 2-34 at the time and I think if anything I peaked too early with a 1-55 '20' in the build up (2-30 pace)

Anyway I'd say using my method a 3-00-00 marathon runner would look at 85-42 for a half as an equivalent run.