I think this formula is better than yours:

Ef=E(R)-Sum(ni*ui); which E(R) is structural energy of graphene and ni are atoms of species i and the ui is the chemical potential of the atoms. DFT is better than gaussian software. It is better to compute this parameter by DFT.

The equation you put in your question doesn't match the stoichiometry of the reaction: formally, you are inserting CHCl into your Coronene/Nanographene slab, not CH2Cl.

The formula that Arezou Zarei brought up is the atomization energy(=decomposition into single atoms), not the formation energy. The latter is defined as the energy required to form the compound from the most stable modification of its elements (i.e. graphite, H2 and Cl2), so choose what you actually want.

If you want the formation energy: Since calculating an actual graphite crystal (=periodic boundary conditions) is probably precisely the thing you want to avoid, I would decompose the reaction as follows:

2Coronene(unfunc) + C2H2Cl2 -> 2Coronene(func.)

All those are close-shell molecules that DFT can handle pretty well with a strong functional. The formation enthalpies of both educts can easily be googled.

The reaction energy ER can be calculated both from the absolute DFT energies Eabs as well as the formation energiesEf:

ER=2Eabs(Corfunc)-2Eabs(Corunfunc)-Eabs(C2H2Cl2)=2Ef(Corfunc)-2Ef(Corunfunc)-Ef(C2H2Cl2)

Now all you need to do is calculated the three Eabs values, get the Ef values of your educts from the literature and calculate Ef(Corfunc).

Edit: just be aware that there are still additional effects. First, mixing DFT data and experimental thermochemistry may cause errors (but you'll get that with any theoretical approach). Second, consider calculating counterpoise-corrected binding energies in order to account for basis set superposition errors.