As a native spanish I was wondering about the words ending in tial or cial, in Spanish they end in cial, for example
English tangential => tangencial versus
English spacial => espacial
grep to the rescue:
grep -cE "[^aeiou]tial$" british-english => 40
grep -cE "tial$" british-english => 43,
conclusion: with 3 exceptions the rule is (no vowel) + tial
grep -Ec "[aeiou]cial$" british-english => 21
grep -Ec "cial$" british-english => 26,
so with 5 exceptions (vowel)+cial.
There must be many low hanging fruit to collect rules of thumb using grep to help you use words correctly. Perhaps someone can give a hint about some of them.
Edit: It seems this rule is well known: https://howtospell.co.uk/cial-and-tial-spelling-rules