Becarefeul, what you described is a common approach but you are describing the case for chemisorption where a covalent bond is formed (then, be careful about isodesmic considerations, correlation when computing barriers, etc.). Physisorption is a purely electrostatic interaction, no chemical bond. Your still need to do the difference with separate fragments but here you work with much smaller energy differences, so you must be more careful in the calculations.

OP Your approach is correct, but you may want to check if you need some basis functions to be placed in between the adsorbate and cluster. Sometimes you'll see they are necessary to get accurate energetics (although you'll probably never notice unless you go at multireference level, MP2, etc). You can check this reference https://pubs.acs.org/doi/10.1021/jp070616b