You need to estimate the extracted RNA concentration using 1 micro L of freshly extracted from bloob sample. Nanodrop will give the value of concentration of RNA suppose 1000 ng/microL that means you now need to take 4 microL of your extracted RNA to get the desire amont of 4 micro g for cDNA syntgesis and so on. In this case you ca use the formula of V1S1 = V2S2 for calculting the exact microL to be taken to get 4 micro g of RNA.