You need quite a bit of info to accurately calculate your required flow at idle.
The calculator in that link also confirms this - you need to give accurate information in the boxes to get sensible answers out.
Using the method I wrote with a VE of 100% only approximates for full load at your maximum engine efficiency point - so your first calculation was a bit off
The throttle is basically a VE reducing device.
Even at full load at 1000rpm, your VE will probably be more like 80%. At idle, you can relate it more closely to manifold pressure - so if you engine idles at 20kPa for example then you can approximate this to 20% VE, and hence adjust your calculation for 100% (5.8ms) by dividing by 5 to get 1.2 (ish).
Engine airflow is one part, injector delay time (or null time) is another. Is the quoted 0.9ms the total time or the effective time. If its the latter then the actual electrical energising time (total time) may well need to be 1.2 or greater, depending on how well you can control the injector, what the fuel pressure is, what your normal battery voltage is at idle and so on.
Lastly there are fuel preparation effects to consider. Engines are different, and if you have a highly tuned NA, you may actually require quite a lot more fuel at idle than you might calculate due to poor atomisation (caused by both larger injectors and low charge velocities due to big diameter ports) and losses directly through the cylinder (cam timing and cylinder design effects). This would actually work in your favour with the bigger injectors (assuming they really do have a good spray pattern and atomisation properties) as this will increase the required pw slightly and gain you some headroom back. For your reference, this is kind of what the BSFC bit in that calulator link is about, although I think they are only using that for the full load calculation, since BSFC is not a fixed number but varies considerably across the speed/load area.
So I'm afraid the answer is there isn't really an easy way to calculate the required pw at idle, as it depends on lots of variables. You can have an estimate but unless you know the details of your engine then its likely to be quite innaccurate.
Hence my recommendation to select your injectors based on your peak flow requirement, and try to stay as close to that as possible. As you've correctly identified, as long as your minimum injection time is greater than that required to give stable (actually repeatable shot to shot) then you should be OK, but the more headroom you have then the better really. If you've calculated that you requre 242cc/min then 460 is really a bit on the big side, and has the potential to bring more problems than it solves.
