Grammar spelling etc

I have been wondering no 8 recently about doubling the last letter of the word as for example

Focus…should it be

Focused or focussed?

There is a rule about this as follows

Is the last syllable of the word is stressed and ends in a consonant then you do double the last letter.

But for focus but the last syllable is not stressed so the past tense will be

Focused

If I have made a mistake please tell me