Take all this with a grain of salt since, as mentioned in another thread, my math skills leave MUCH to be desired. I have tried making my own and was never successful. I also bought a HF cheapy jig. It worked ok after I made some adjustments to it but still lacked what I was after. Currently I have a Dowel-it 2500 which is far superior to the HF. All that said, I still haven't drilled a bow with it so I can't attest to how well it works for that but all the testing I did with it proved to be accurate within about a half MM. I am not sure what that is with imperial measurements, I just happen to have a ruler with metric on one side and it was easier to read than the other.