I used the following formula to calculate the ohms:
Basically though, if you want 20mA, the resistor should be ([supply voltage] - [led voltage]) / 0.02. Or 50*(supply voltage - led voltage).
So I calculated: 5V - 3.5 (led voltage) / 0.02 = 75 ohms
=> P = V^2/R with this formula I get around 0.33 watts?
Is this not correct?
About the 0.25w items on that site: what I don't understand is why there are several items available
starting from 1 W upwards. Is not this one the actual resistance value?
best
yves