dvd is right, it's not very scientific. Recoil can actually be calculated using this mathematical formula.

CALCULATING FREE RECOIL
The most practical measurement for us is free recoil energy in foot-pounds. To find this you must know the weights of gun, bullet and charge and the velocity of the bullet. The three step calculations (taken from The NRA Firearms Fact Book) are:
1. I = (WB x VB + 4,000 x WC) / 225,400
I = Recoil Impulse
WB = weight bullet (grains)
VB = velocity bullet (fps)
WC = weight charge (grains)
4,000 = assumed velocity of charge in fps
225,400 = constant to adjust units

2.VG = 32.2 x I / WG
VG = velocity gun (fps)
32.2 = gravity
WG = weight gun (lbs)

3. EG = WG x VG squared / 64.4 (2g)
EG = free recoil energy (ft/lb)
VG = velocity of gun (fps)
64.4 = 2g (acceleration of gravity)
To make these calculations all you need are accurate scales (for both pounds and grains) and a chronograph.

If you do your experiment as you have described, you will have too many variables and you will not have very good control. Sorry to burst your bubble. But if you have accurate scales and a chronograph, you could do your experiment based around this formula. Just be sure to use the same gun every time and minimize your number of variables by cleaning your barrel after every shot, shooting at the same elevation every time, and make sure the weather is always the same (i.e. wind, barometric pressure).