It's been a while, but here goes!
We can assign a value to the original plenum volume, say it's Z
You measure the internal volume of spacer (sand/oil/geometry/whatever method) and it turns out to be 50cm3.
From the 12% increase figure quoted, we can say the original volume (Z) + the 50 you measured = original volume (Z) +12%
Which is just the same as Z + 50 = 1.12 x Z
This can be solved for Z, which is the original volume of the plenum.
SOLVING Z + 50 = 1.12 x Z
Move all terms containing Z to the left, all other terms to the right.
Add '-1.12Z' to each side of the equation.
50 + Z + -1.12Z = 1.12Z + -1.12Z
Combine like terms: Z + -1.12Z = -0.12Z
50 + -0.12Z = 1.12Z + -1.12Z
Combine like terms: 1.12Z + -1.12Z = 0.00
50 + -0.12Z = 0.00
Add '-50' to each side of the equation.
50 + -50 + -0.12Z = 0.00 + -50
Combine like terms: 50 + -50 = 0
0 + -0.12Z = 0.00 + -50
-0.12Z = 0.00 + -50
Combine like terms: 0.00 + -50 = -50
-0.12Z = -50
Divide each side by ‘-0.12'.
Z = 416.6666667
So if the volume of the spacer is 50cm3 and it delivers a 12% volume increase, the original volume of the plenum is 416.67cm3
Alternatively, you can put the original Z + 50 = 1.12 x Z equation into this website:
https://mathway.com
and just swap the 50 I made up for the demonstration for the actual value you measure inside the plenum spacer. The answer Z will be the volume you are looking for.
In short, original plenum volume = your measured spacer volume divided by 0.12