You gain 20 fps for every 10#s of draw weight added.

What are the variables?

Poor way to guess an increase.
It would be better to use a percentage than to use a 10 Lbs = XXX increase in speed.
add 10 Lbs to a bow shooting 10 lbs already...and you' ll likely gain more than 20 fps. Add 10 to a bow shooting 80 Lbs, and you will gain less than the bow that was shooting 10 Lbs.
Draw length will also be a factor, as will arrow weight, cam profile, frictional losses, ATA, and brace height.
If shooting a VERY light arrow, you will be getting closer to to bows maximum string speed (i.e. dry fire) shooting a very heavy arrow moves you farther wawy from that speed.
ATA, has a larger effect on the V-max of the string (Not the PERFECT use of the term, but it fits here) brace and cam profile effect the acceleration curve. Friction effects everything.
Now, when you start narrowing down the criteria. Say, add 10 Lbs to a hunting compound (32-38 ATA, 6-8" brace, shooting a 6-7 gn/Lb arrow,etc) you might be able to get a closer generalization.
Basically, if a added 10 lbs to my compound shooting light arrows, I would expect different results than say someone shooting a longbow with really heavy arrows.

Something else to consider is you cant shoot the same arrow . Put 10 lbs on your bow and your going to be underspined .

When your shooting the same arrow that falls within a ten pound increase or decrease for spine I' ve found the 20fps rule of thumb to be very accurate give or take 1-2 fps here or there.

Guys, I was thinking a 500 grain arrow is a 500 grain arrow. I' m sure one could properly spine a 500 grain arrow for both draw weights.

True! For a 500 gr arrow. But when you start talking about an under spined 300 - 350 gr arrow that comes off the string sideways it plays havoc with arrow speed!