When bringing in a new cofounder it's important to give them an opportunity to earn their equity. Typically this means a vesting schedule over a certain period of time. For example you may decide on an opportunity to earn 10% of the company over 4 years with a 1 year cliff. This means there is no equity earned for the first year. After the first year they would immediately receive 2.5% (for the previous years work) and then a prorated amount (.2%) each month after until the end of the fourth year. This helps everyone's incentive stay aligned.

Now, in your situation it's important to understand why you want a CTO cofounder. From your description it sounds like you already have everything you need. Think about this first.

If you still want to find a tech cofounder then consider the following when making an offer:
What you would want if you were them?
What are they giving up to join you?
What will build a long term relationship?
What is the company worth without them?

The amount of equity can be supplemented by a salary, amount of time committed, or other compensation. Without knowing the specifics it's very challenging to give an exact % answer.