I was in the shop this weekend and was trying to figure out the math for the settings on the Festool Domino to space the mortices evenly and centered across a cabinet front to back. After a bunch of trial and error with my math, I gave up and got close enough and cut the Domino slots for most of the cabinets before my shop time ran out. It was still bugging me, so I figured it out on the plane on the way to work.

Very simply, we need to subtract the width of all mortises created from the width of the workpiece and divide what's left by the number of spaces we will have between the mortises. Of course, there are some complications to this method that need to be resolved.

1. When you use the paddle stop on the Domino for the first mortise, the distance between the edge of the workpiece and the first mortise will be different from the distance between the remaining mortises.
2. When using the paddles or the cross stop, the setting on the domino shows the distances between the edge of the workpiece or previous domino and the center of the next domino to be cut, so we need to add 1/2 the width of each mortise to the setting.
3. Mortise width is a combination of the width setting on the Domino and which cutter is installed.
4. There is a "no man's land" of settings between the built-in pins/paddles on the Domino and the cross stop. The lowest setting on the cross stop is 100mm or about 4".

What you'll need to know to start
B = Domino cutter bit diameter. These range from 4mm to 14mm depending on the model of Domino joiner you own.
S = Setting on the domino jointer. There are three options on the DF500 which are 13mm, 19mm, and 23mm. The DF700 has two options 13.5mm and 16.5mm.
L = Length of the run you want the Dominos spread across
M = Number of dominos you want in the length
P = Pin/Paddle setting for the first domino. On the DF500 this is fixed at 37mm on the DF700, there are three options which are 20mm, 37mm, and 50mm

Stuff to Calculate
W = Mortise width created by the Domino. Determine W. Which is the mortise width created by the domino. This will be the width setting on the Domino Joiner + the diameter of the bit or W = B + S
Looking at the diagram below, you can see that center portion in yellow comes from width setting on the Domino and represents the swing of the bit center to center. the total width of the mortise then adds the radius of the bit on each end.



This diagram assumes a 5mm cutter with the width setting at medium on a DF500

O = Offset from the edge of the workpiece to the far edge of the first mortise. There are two options you can use when starting the run of mortises

1. Use the built-in paddle at one of the three settings which sets your first mortise centered at 20, 37, or 50 millimeters from the edge. Determine the distance from the edge of the workpiece to the far edge of the first mortise which will be your paddle offset + 1/2 of the mortise width. O = P + (W/2)
2. Use the setting on the cross stop for all of the mortises which sets the first mortise the same distance from the edge as the distance between all the mortises. In this case, O drops out of the formula so there is no need to calculate it.

Now we're finally ready to calculate the setting on the Domino itself. Note that if you get a value less than 100mm or you need to reduce the number of dominos or if the setting is > the max setting on the cross stop you will need to increase the number of dominos being used.

If you are using one of the built-in pins/paddles as a starting point.

Cross Stop Setting = (L-(O*2+W*M))/(M-1) +W/2
Using the cross stop setting to start your first mortise rather than the paddle only changes the number of spaces between mortises which is accomplished by adding a mortise spacing on either end rather than subtracting one and ignoring the offset.

Cross Stop Setting = (L-(W*M))/(M+1) +W/2
Finally, the cross stop setting is in whole millimeters and you might end up with a decimal in the cross stop setting. Round this down. If you are trying to make truly symmetrical parts, then start half the mortises from one edge and the other half from the opposite edge.

Below is an example using both methods.