can anyone advise

[linnsound]

our existing lighting circuit consists of ten lamps and i'm thinking of adding a further 2 wall lights.What calculations should i be doing to see if the circuit is capable...If i assume that all lamps are 100w, and using 1.5 T&E do i simply calculate maximum demand? ie 66% of 5A being 3.3A...as circuit protected by 10A type B rcbo am i good to go or am i missing a more fundamental calculation?

thanks