When you want to space fasteners evenly along a workpiece, here’s an easy solution for calculating those distances: Take the number of fasteners you plan to install, add one and divide by the length of the joint.
For instance, if the joint is 36″ and you plan to install five screws, 5 + 1 = 6; 36″ divided by 6 means the fasteners should be spaced every 6″ – at 6″, 12″, 18″, 24″ and 30″. If you want the first and last screws to be inset from the ends of the workpiece a certain distance, subtract these two distances from the joint length first, then use the first screw as your starting point and apply the same formula for the remaining screws.
Sooner or later you’ll end up with fractional measurements, but the principle is the same. To skip the fractions altogether, use a metric tape measure instead.
– Serge Duclos