evry plant/herb will have active chemical compound. first quantify this compound in the total individual herbal extract. next, calculate the amount present in the herbal formulation will clearly indicate the amount of herbal extract present in herbal formulation, HPTLC  instrumentation will be helpful in this experiment

I recommend you the see the following link and paper

Good luck

http://ianbreakspear.com.au/2014/09/14/dosage-calculations-extract-equivalency-in-herbal-medicine/

Its really debatable if extracts can replace the formulations as formulations are much crude and contain more phytoconstituents ,while extracts tend to contain only the active principles.It has been proved that not all active principles are known and efficacious .

But still if you wish to go for it then first conduct a thorough literature review for the respective active principle and as suggested above you may use  chromatographic methods of separation(HPLC/HPTLC) .Isolated compounds can be further investigated for their structure activity relationship.

The need of use of Herbal Extract in Herbal formulation is due to reducing contamination e.g. Pathogen,Havey Metals, Insecticide/Pesticide residue etc.As per Active drug is concern I agree to Dr Jyoti. Only isolating few compounds from plants which we say are pharmacologically active is not true methadology of formulation.But Water - Hydro alcoholic - Alcoholic Extract  or combination of all three can be replace herb powders most of the times. Now the question is how to quantify concentration of extract?For that we have to use some markers and HPLC/HPTLC methods. Analyzing marker compounds in Raw Herbs and Herbal Extract we can easily find how much time extract is concentrated than its source herb. 

Dear Deepak

Yes, agree we can quantify the herbal extract, first we have to see the what kind herbal formulation such as single herbal formulation or multiple herbal formulations, further go to the available literature on that herbal product manufacture things. Each plant/ herbs having their own chemical bioactive molecules/ compounds. Then Isolate the compounds using available methods such as preliminary HPLC/HPTLC etc. Characterize the chemical compound and their structural activity relationship. Further analyzing suitable marker compounds in Raw Herbs and Herbal Extract, we can easily find how much time extract is concentrated than its source herb.

I am understanding the question differently than the answers so far address. It sounds like you are asking about quantifying the dosage of an extract used in a formulation by comparison to the quantify of botanical raw material that would be used (traditionally). If that is the question, then, if the extract has been manufactured in such a way that its composition is representative of the composition of the starting material, then one generally used the drug-to-extract (DER) ratio proportionately in the formulation. If the formulation calls for 7 grams of herbal raw material and the native extract results in a DER of 7:1 (w/w), then 1 gram of 7:1 extract would be used in the formulation. I'm not sure that is what your question is meaning however.

for quantification of a herbal extract to be in herbal formulation, first the herbal extract to be formulated, should be standardized according to its main active constituent (s), then the calculated bioactive amount of the standardized extract can be formulated for therapeutic used.