My dear that is easy, you can take 10 µl from stock prime (which diluted depended on procedure of company some reference 150 µl free DW add to powder primer), and add to 90 µl free DW.

You can use the formula C1V1 = C2V2 so if the pcr is in 20ul that is v2 and the wanted concentration is 15pmol and the concentration of the stock is 100pmol (c1) then

c1v1 is 100xv1 =c2v2 which is 15x20 so100v1=15x20 or v1= 15x20/100 = 3ul

similarly for a needed concentration of 10pmol 100xv1=10x20 so v1=2ul

As explained above by my colleagues; If your primer stock solution is 100pmol/µl (=100µM), then for a volume of 100µl primer working solution preparation, you can proceed as below:

1-[15pmol/µl =15µM ]: add 15µl of the stock solution to 85µl of water,

2-[10pmol/µl =10µM]: add 10µl of the stock solution to 90µl of water,

3-[5pmol/µl =5µM ]: add 5µl of the stock solution to 95µl of water.

Now, the final concentration will depend on the PCR reaction volume:

e.g: for a standard 25µl volume, the final concentrations will be 0.6pmol/µl (=0.6µM or 600nM) for 1; 0.4pmol/µl (=0.4µM or 400nM)for 2; and 0.2pmol/µl (=0.2µM or 200nM)for 3.

Use the formula C1V1=C2V2

Use the formula C1V1=C2V2 or Google "C1V1=C2V2 calculator."