I started using a Bahtinov mask last December and found it invaluable. At times I found it difficult to judge the exact point at which the central spike was midway between the other two spikes.
After some experimentation I came up with this design.

Instead of a central spike, this mask generates two 'X' shapes which are symmetrical when perfect focus is reached. Any error in focus causes an asymmetry which is very obvious to see. The shape of the asymmetry indicates which way to alter focus.
Please have a look at this webpage for a clearer explanation.
http://www.btinternet.com/~gfcarey/careymask.htm

I welcome your feedback and/or questions.
Thanks,
George