At one level, co-founders should be viewed as early employees who can forego cash, and you can use the above methodology to arrive at equity levels given that.
However, co-founders are more than that - they should be your partners in building out the business. If you are strong technically, you want to find a business co-founder who is as good at the business as you are in the technology. They are your true partners.
As such, the range for co-founder equity can vary between a VP-level hire + a couple percent premium to equal equity to you.
If your co-founder is joining you after you have invested significant time in the business, you should separate out the equity you deserve for having built the business to date, from the equity you will get going forward. The latter piece should be equal for a true co-founder you want to bring on. Dropbox started like this. Drew got a premium for all the work he had done to date, but brought on a co-founder and they split the remaining equity. This is not an exact science.
The title "co-founder" is applied liberally. It typically ranges from at least 5% to up to 50% of a business. You can also give someone a title as a "Founding Engineer" for a first time hire.
You can always up someone's equity later if they outperform, it's very difficult to reduce. So you can also establish a check-in point 6 months post start to reassess the equity packages if there is disagreement.