Sounds like you need a decent gear textbook or handbook, of which there are many. I would suggest Dudley's gear handbook, second edition, page 4.7, where you will find all the necessary equations. I could easily reproduce them, but to encourage you to get yourself a textbook, I'm not going to. However, others probably will anyway! The thing to be aware of is that there are three contact ratios commonly referred to - transverse, axial, and total, and there is also what is known as the overlap ratio (see Dudley). But I'll give you one tip. The total helical contact ratio should be at least 2, although ideally most gear designers prefer to use integer overlap ratios such as 1 ,2 or 3, since this is usually thought to give smoother meshing. However, I have seen designs with overlap ratios less than 1 where the designer was aiming to minimize axial thrust, and they seemed to run quietly - it really depends on how heavily loaded they are.
I agree with EnglishMuffin whole hearteldy. Most designers would want to keep the ratio around the 2 mark. Axial contact ratio needs to be at least 1 otherwise your dealing with LACR gears (low axial contact ratio) if this does turn out to be the case, try to increase face width or problems can creep in with the tooth bending stress and the gears may get noisy and generate knocking.
