How Much Current for Line-Following Sensor QRD1114??

[richardv2]

So I ordered a bunch of QRD1114's for a line-following robot to use as black/white sensors.

Most schematics I've ever seen put a 270 ohm or 330 ohm current limiting resistor in a (regulated) 5-volt setup. So my math would be:

I = E / R = 5v / 270 ohms = 18.5 ma
and
I = E / R = 5v / 330 ohms = 15.1 ma

..and the data sheet for the QED1114 says the max current for this device is 50 ma, so it looks like I'm seriously underpowering the LED.

I = E / R = 5v / 100 = 50 ma
and I verified that a 100 ohm resistor gives me the full 50 milliamps.

The problem: I probably don't want to run these LEDs maxed out. Don't want to waste power - might burn out the LED faster, etc. So I want to reduce the power until I do *NOT* loose sensor sensitivity. I plan to run experiments at 50 ma, and down in increments until I see some change or some degradation of my results. I also see a problem that if I get it "just right", I could be low enough to get blown out by higher than expected ambient lighting if my light shielding isn't perfect.

The question: How would you conduct the experiment I mention above? What am I looking for to tell me I have the right LED current?