First, determine how many escape wheel teeth you're going to span. If you're making pallets for an extant escape wheel, count them. Then draw lines from the center of the wheel to the tips of the two teeth.
Draw perpendiculars to the lines you've just drawn at the the tooth tips. The point at the top where they meet will be the location of the pallet arbor. You've now determined the distance between the escape and pallet arbors. (In an extant escapement, the calculated distance will probably not be exactly the same as the actual distance, but it will be close.)
From the same tooth-tips, draw two new lines at 35 degree to the right of the perpendiculars, shown here in red.
A circle drawn tangent to these two lines at a and b has the radius d. This is the distance needed for grinding the impulse face angles.
If you're making pallets from scratch, you'll use the measurement shown here in red as the length of the outside arc of the pallets. You do have to subtract half of the drop from this measurment to get clearance. The measurement is the same on both right and left pallet arms.
The thickness of the pallet nibs is half of the distance from tooth tip to tooth tip, minus the drop clearance.
Outside pallet arc = arm - 1/2 drop.
Pallet thickness = 1/2 tooth pitch - drop.
Inside pallet arc = outside arc - pallet thickness.
The escapement with all the dimensions on the right. There is 1/2 degree of lock shown. (Click on the drawing for a larger view.)
Not too hard, was it?
Another escape wheel and another set of pallets. The escape is 1.5" in diameter and has 30 teeth.
If you're making pallets from scratch, you'll use the measurement shown here in red as the length of the outside arc of the pallets. You do have to subtract half of the drop from this measurment to get clearance. The measurement is the same on both right and left pallet arms.