Some folks make an assumption that the FID trace is effectively a mass trace -- and it comes fairly close for most compounds of interest to someone like a flavorist. The FID essentially counts carbon atoms. And highly oxygenated compounds (like sugars) will never make it down the column. So, you can expect a major portion of the mass of the molecule making it to the detector to be carbon. (Remember, we are looking at the kinds of things in flavors, and aside from H2S and NH3, there is some carbon in just about every molecule that we would want to smell - and some that are of natural origin that we do not want to smell.)
Taking this a step further, the total response on the MS roughly follows that of the FID. So, some will use the Total Ion Chromatogram (Or total ion current or TIC for short) from a full scan acquisition to estimate percent composition in a mixture. And there are rules for adjusting response factors for compound classes. I recall seeing this in some class slides for a flavor chemistry class that were posted on line several years ago. I believe the class was by Dr. Gary Reineccius (a highly respected food and flavor chemist) from the University of Minnesota. And if I can find the slides, I’ll post the link.
Depending on what you are doing, this works quite well enough. If you are using the TIC trace from an MS to estimate that a fragrance mixture has 5% geraniol in it, you are probably good enough. If you use this technique to show that there is 5 ppb of DDT is a soil sample -- you won't have much credibility. (Actually you probably wouldn't find the peak in the TIC anyhow.)
Bottom line - the sophistication of the tool required depends on the task at hand.