How can I choose between "-se" and "-ce" for the spelling of the word ending pronounced /s/. For example, both "sense" and "science" have the ending pronounced /s/.
There's not very many patterns that would help in most cases. Small patterns exist but you are better off simply memorizing the spelling of common -ce and -se words.
For example - the c in -ace or -ice at the end of words is almost always pronounced /s/, whereas the s in -ase or -ise would be usually pronounced /z/. So use -ce in that situation.