Intracellular Lypid layers quantification (advanced question)

I read an estimate in a wikipedia article saying that the plasma membrane of an average cell contains about 2% of the membranous components of the cell (ER, golgy, mitochondria...).

Sadly, it wasn't cited. so I have a few questions.

Can you refer me to a nice source, I can't find any for some reason. Is this even true? (it sounds plausible). How variable is this?
How is this evaluated? (I can think of a few ways but just to be sure)

Of that 98% percent of intracellular memranes components, how much is exposed to the cytoplasm directly (as apposed to being sequence layered machinery like most of the ER and golgy).

cites please.

*I know it's a weird question, I'm trying to estimate something about the theoretical intracellular capacitance of the intracellular components of a cell body.