Setting gain on amp

I would like to set the gain on my amp but I am confused.  I would be using a multimeter and will be using the E=(P*R)^.5  BUT I am confused which number goes where because I will be bridging the amp.  In JL's tutorial, it says that I have to half the load and then double the voltage at the end.

i have a Rockford Fosgate 500a2 and 2-HE2 10" subs wired for 4 ohms.  The amp is rated at 125 W X 2 @ 4 Ohms RMS,250 W X 2 @ 2 Ohms RMS, and 500 W X 1 @ 4 Ohms Bridged RMS 

If I use P=500 and R=2 (half of 4) I get 31.62 which doubled is 63.34 Volts.  Seems kind of high? Thanks.

Assuming the load you are placing on the amp is 4-ohms TOTAL, your correct target is 31.62.  The JL tutorial is refering to using their charts which do not include the bridged power rating of the amp.  Since you know yours is 500 watts @ 4 ohms, 500 X 2 = 1000, sq. rt. of 1000 = 31.62 volts.  Remember to use a test tone of about 50Hz and to be sure your head unit is set below it's clipping threshold.

Your amp cannot handle anything lower than 4-ohms in bridged mode.

sorry to bring this back up. but, i think I have finally got it and want to double check.  the 31.62 reading, i should be doing that to each channel correct?  if i were to check it in mono, it will rear twice that whic h is 63.34

someone else tells me to do it this way:

"V = square_root(500*4)

V = 44.72"

Look again at the specs you listed in original post:  "The amp is rated at 125 W X 2 @ 4 Ohms RMS..."

125 X 2 = 250

The amp is rated at 250 Watts at 4 ohms.

Arithmetic.

i do believe your 63.34 reading would be correct for the amp in the bridged mode....31.62 per channel i do believe.