Lead came is normally cut to meet up against the adjoining cames. This provides a neat joint that will carry the solder without needing to fill gaps. The easiest joints to make are those at right angles. You can measure or estimate the amount that the came must be shorter than the glass, but it is easiest to use a piece of came the same size as will be passing the piece you are cutting (gauge came). You centre the gauge came on the cartoon cut line near the joint. Using your lead knife you can extend the line of the side of the gauge came to the piece you will be cutting. The mark you make with the lead knife can then be used to guide your cut of the came, as you take it away from the glass.
Those joints with angles have the came marked and cut in the same way as for right angles as the gauge came will give you the correct angle to cut.
Always remember when leading that you lead to the cartoon lines not to the glass. If the glass is short, use the cut line to place the gauge on, not against the glass. If the glass is too large, adjust its fit.