You raise a great question, as most don't realize that the mechanism of action Lasix exerts goes beyond just diuresis. The decrease in venous tone and increase in venous capacitance within about 5min of IV administration is just as important (and potentially exacerbates hypotension). You may have a hard time finding exact numbers to correlate with the risk/benefit of giving lasix, but going back to diuresis for a sec I would like to answer your question with one of my own that could help guide your clinical decisions.
What pressure is required to perfuse the kidneys and allow diuresis to occur?
The Renal Perfusion Pressure (RPP) must be maintained at a level which allows the kidney to function. This means different things to different people depending on underlying kidney pathology, chronic hypertension, drug use, etc.. A good guesstimate in a healthy person is that if they have a Mean Arterial Pressure (MAP) of somewhere lower than 60-70mmHg you can expect that they are not adequately perfusing their kidneys. Therefore their Glomerular Filtration Rate (GFR) will drop and they wont be making much urine. For people with renal pathology or chronic HTN their kidneys will be accustomed to even higher pressures and may require even higher MAP's.
Long story short, don't count on Lasix as an effective treatment in the hypotensive acute pulmonary edema patient. If you are worried about their blood pressure, you should be more worried that the drug can't exert its mechanism of action at all.
clear as mud?