In copper foiling, a considerable amount of work goes into getting the pieces to fit with just enough space to accommodate the copper foil and a thin space for the solder fin to join both sides. This of course, promotes consistently narrow solder lines without the solder melting through to the opposite side.
Grinding to pattern
Many times it is necessary to grind to fit pieces together with this degree of accuracy. Those who draw onto the glass or stick pattern pieces to the glass, often grind to the template or the drawn lines. This can lead to inaccuracies in relation to the cartoon.
The object in scoring and breaking the glass is to be as accurate as possible. This reduces the amount of grinding required. It saves time. It makes the whole process easier. Still, we all have to grind relatively often.
Grinding to cartoon
In my view, when grinding to fit, you should be trying the piece out against the cartoon, rather than the template or the drawing on the glass. This will tell you how well the current piece fits in with the rest of the pieces you have already fitted to the cartoon.
The cartoon drives the assembly of the whole piece. Thinking you can just make small adjustments as you work along, creates increasing difficulties in making the whole fit together. If you follow this principle of fitting to the cartoon, you are judging the accuracy of the piece against the cartoon lines, rather than any template or drawing on the piece of glass. This means that the fit will be correct and the whole will go together with the minimum of difficulty.