The question is: if you run a half at flat heart rate, does you pace stay flat, with an equal or slightly negative split? For me there is a slight fade, which I offset with say a couple of % either side of 88 % in thirds, IIRC. I don't often run a half, but here's my half pb from 2011 with a trending up, mini-marco HR. I could have been too generous here, since I seemed to have quite a bit left for the last 3 km
Edit: here's a better one from last year where I aimed at 87, 89 and 91% for a trail half.
I've been using this method for a few years. I absolutely agree it's better to run after the HR and see where it comes out. The only downside is that one is not sure exactly how fit one is (but one could get that from training splits) so it's hard to estimate finish time.
If you look at my Copenhagen marathon 2014 you can see that it roughly falls into the prescribed MARCO thirds, but that it was hard to hit the goal for the last third, as Phil says. This upside of this is that the first half to two thirds is not difficult Even though I was not fully up to target for the last 10 km, I ran over a 6 min pb in this race compared with the previous autumn. Coincidentally, Copenhagen 2014 was the same time as I ran in Boston 2014 using Marco, to the second!