Ok i'll try this
But do the power have do be the same ? or can I power the led strip with ma 5V15A power and the raspberry from a phone charger for exemple ?
remember that you can do it both ways like you mentioned, but extremely important >>> ground the GND PINS (at least 2 pins) of the PI to GND of ledstrip and GND of PSU!
also mentioned as 0v / V- / GND its all the same.
stilll...... your PSU is not to be trusted ( powerbrick) or your multimeter is not correct > you have a powerdrop at only 30 leds from 5,17 volts to 4,7 volts?
something is not quite right here, i told you before and you are not going to response on that.
You can put separate phonecharger on the PI, but i am not vouching for that to work.
goodluck.