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Measure current through the diodes without the resistor, and how much higher than speck the B+ is the resistance will be about equal to the voltage over current. But adding the resistor will change the behavior of the circuit some so you may want to tweak the value to get the voltage closer to speck. You want to use a power resistor with a rating in watts that is equal to or greater than (greater is safer) the planned voltage drop across the resistor times the current through the resistor.
Some folks neglect the resistor if the increase in B+ voltage from the silicon diodes is within 10 or 20% of speck.
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