I was thinking the same and did build it that way first but then I started to over-think it so just set it all at 190, figuring I can come out of ERG every now and then to either push a bit harder, or take a rest. I can always use the % change too to tweak the power as I feel fit as the ride goes on. The problem with a long ride is knowing how you'll feel 90 mins in, so what I set prior to the ride may not match how I'll feeling later - this is where I ca just come out of ERG or increase/decrease power on the fly.
Probably worth saying I do try to vary the cadence on the ride too, standing up at times, spinning a bit faster, slowing down etc, just to give me a bit of variation.