IMO macmillan is handy for 'shorter' and comparable distances eg a 10km vs Half Mara, Half Mara vs Mara. One thing you have to take into account for ultras is the surface, terrain, profile. You mention it is flat, but presumably softer under foot. Road marathons are painful, muscle soreness from pouning on pavement and tarmac. On softer surfaces you'll be able to go a lot further without suffering the same soreness and fatigue. Of course pacing can be affected by that change in surface as well as other factors eg gates, stiles, checkpoints/gear change, etc.
Personally Id say run it on feel. Build through the first few miles so you dont set off with faster runners, then settle into a 'comfortable' pace. You could add 30-60secs on your mara pace which you said was comfortable ie 10:30/M and see how that goes. another thing you can do is take the HRM and just ensure you are at least 5bpm below your LT once up to speed. That would ensure you dont get carried away.
I'd just go out and enjoy it, keep steady and take experience forward to the next race. I think Ive done about 6 ultras now and still keep increasing effort and pace for each. I wouldnt want to break early in an ultra.