Some people look at “average” 1, 2, 3, etc., but that discounts the fact that there are a limited number of ace pitchers, meaning that some teams don’t have true No. 1’s or 2’s in their rotation. I think the “ideal” rotation pitchers are those associated with good pitching staffs. Most Astros fans think the team should aspire to a good, not just average, pitching staff. Therefore, I will use several very good NL pitching staffs as examples. (I used fangraphs, plus some judgement for this.) I show ERA below…

Cubs: 1) 1.77 2) 2.96 3) 3.91 4) 4.09 5)4.43
Mets 1) 2.53 2) 3.72 3) 4.18 4) 4.22 5) 4.50 – 5.61
Dodgers 1) 3.16 2) 3.24 3) 3.73 4) 4.30 5) 5.09

In this unscientific approach, the following ranges seem about right to me, with some margin or range above the high side for each rotation number:
No. 1 ) 1.77 – 3.16 (I would go as high as 3.5)
No. 2) 2.96 – 3.72
No. 3) 3.24 – 3.91 (I would go as high as 4.10)
No. 4) 4.09 – 4.30 (I would go as high as mid-4’s)
No. 5) 4.43-5.61

Sure, these are “idealized” rotation ERAs, and guys may not hit the target every year, but I’ll go with it (for NL teams).

Another point is that ERA isn’t the only issue for rotation spots. I think the expectation is that the No. 1 and No. 2 will be “hosses” and pitch 200 innings each. Again, that doesn’t always happen because of injuries, but it’s what you are looking for. The 3 and 4 starters hopefully pitch long enough to get a lot of quality starts and 150-175 innings. The No. 5 starter: you pray for rain.