While I've done TLC before, like MB, I've never used it quantitatively. In case it might help, here's a google book source describing a method like MB says of scraping it off and using another technique (a colorometric technique):
Introductory Practical Biochemistry - Google Book Search
And, here's a whole book on Quantitative Thin-layer Chromatography:
Quantitative Thin-layer ... - Google Book Search
~modest
