I think most good brewing software covers this situation reasonably well. Brewers Friend lets you input your volume at the start of the boil and the target volume going into your fermenter (which would include your topoff water), and it will do the appropriate IBU adjustments from there.
As you said, there are essentially two factors affecting your IBU calculations here:
- Lower utilization because you have a higher gravity boil
- Dilution of your IBU's when you top off
At 30 IBU's this doesn't pose too much of an issue, but if you're shooting for a super hoppy IPA you could start running into issues where your concentrated wort is maxed out on IBU's somewhere in the 100IBU range, and depending on how much you dilute it you could end up coming out way lower than expected/calculated.
The IBU calculations don't have an upper limit, so it could look fine (i.e., 150 IBUs calculated in your initial wort diluted 1:1 gets you to 75 IBU's), but your actual wort may actually be in the 80-100IBU range and would dilute down to 40-50 IBU's.