Relay series resistor

[Miyagi_83]

Not sure if I'm posting this in the right place.

Anyway, I came across a thread by joecool85 where he wanted to use a relay for switching channels on his amp. To cut a long story short, it was necessary to lower the voltage needed for the relay.
J M Fahey suggested the following:

The series resistor is needed to bring those 19V down to needed 12V.
Value?: measure the DC resistance of the coil, and multiply it by 0.58 .
Example, if your relay coil has 1200 ohms, you'll need a (1200 * 0.58)=696 ohms.

My question is, why 0.58? At first I thought it was because it's the ratio of needed voltage to available voltage, but
12 / 19 = 0.62
Pretty close, and most probably good enough, but not exactly that.
So, what am I getting wrong?

[Tassieviking]

I have no real idea but it might be because a coil has impedance that acts different then a straight resistor ?
Just like a speaker that is rated at 8 ohm Impedance measures lower in DC ohms with a multi-meter.

[g1]

Not sure how he got that number, magic?  Or maybe I'm just not on top of my game right now.   
But in any case, pick an example number, solve for current, and the added resistor will drop about 7V, leaving 12V on the relay coil. 

[Miyagi_83]

Not sure how he got that number, magic?  Or maybe I'm just not on top of my game right now. 
But in any case, pick an example number, solve for current, and the added resistor will drop about 7V, leaving 12V on the relay coil. 

Yeah, that's what I've done a few times as an exercise, use Ohm's law to find the value needed. The ratios I got were pretty close. I mean, using Mr. Fahey's number the calculated voltage values were good for the relays I had on hand, so perhaps I'm splitting hairs on this one?

[g1]

First off 12/19 is closer to .63 than .62  .
Then, that 63% is of the total, the relay resistance plus the series resistor.
So that's why it can't be used applied directly to the relay resistance.

[Miyagi_83]

First off 12/19 is closer to .63 than .62  .

Whaaaaa? How did I get that one wrong?  You're right ofc.