I have done a good bit of training on an ergo at the gym. I have done long base rides, interval training, and various fitness and HR tests to see what's up, etc.
I think it is very hard, damn near impossible, to hit your real max HR by yourself on a machine.
I think the max I have seen on the ergo is around 176 or 177, and that is doing all kinds of ramp-up tests, intervals, etc.
In the other hand, out riding the bike with other guys in better form than me, I have hit 189 on long, severe climbs, when faced with the posteriors of my friends.
I am 43 by the way.
I would not even be surprised if I couldn't surpass even that HR, if I really got mad, or in a race, etc.
I would suggest that you do your best on the ergo, give it your all, really bust a nut; but treat the number you achieve as a conditional HRmax, and realize that in the heat of battle, you may end up quite a ways above the mark you have set in the calm of the gym.
Still and all, you gotta start somewhere, and a start is better than nothing at all, eh?
Whit