Well - normally a "serving size" listed on a can of "whipped aerosol cream" such as Reddi-Whip is going to be 30 ml (2 [US] Tablespoons). A cup [US] would be 237 ml - and 100 ml would be 0.42 [US] cup.
Assuming that the bowl you are using has straight sides - you can calculate the volume of "whipped cream" by measuring the diameter of the bowl and the depth of the cream added:
Volume = (pr^2) x height (where pi = 3.14)
If you make your measurements in inches - you can convert this to ml:
Volume (in^2) / 0.061 = ml
Of course - if the sides of the bowl flair out in a straight line, you'll need to calculate for the volume of a segment of a cone - and if the sides of the bowl are curved then you have to calculate the volume for a segment of a sphere.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain