A Toe-in of 1 mm is recommended to keep the tires more stable going straight so that any slack/flex in the steering is taken up by the rolling resistance and rider's weight.
To measure toe-in (also known as tracking) you can simply use a tape measure from rim to rim at front and back
Ackermann steering geometry
The intention of Ackermann geometry is to avoid the need for tires to slip sideways when following the path around a curve. When a tadpole trike goes around a corner, it turns around a point along the line of its rear axle. This means that the two front wheels will have to turn through slightly different angles, and the inside wheel turns through a greater angle than the outer. Ackermann geometry is simply steering which achieves this, keeping each front wheel at the correct angle, through the whole range of the steering motion.
Look at the VELOKS MK3 from the front and you'll notice that the kingpins slope outwards.
The idea is that the kingpin “extended” axis meets the ground at or near the contact point of the tire - the so-called “Centrepoint”.
Then if the wheel hits a bump, the forces from this impact will be in line with the turning axis, so no torque can be exerted which might impact the steering. Also, if just one of the front wheels is braked, or the two front wheels are braked unevenly, the forces will pass through the kingpin axes and not affect the steering.
To achieve this, The VELOKS MK3 kingpins has an angle of 17.4 degrees.
Just like a two-wheeler, a tadpole trike's steering needs to self-centre if it's to handle well, and especially to be stable at speed. This is achieved by inclining the kingpin axis. This inclination is in a plane at right angles to the Centrepoint steering inclination.
The VELOKS MK3 has a caster angle of 15 degrees, which means it is very stable.
The Camber angle of the front wheels, is the deviation angle from 90 degrees of the side of the front wheels and the ground plane.
A neutral camber (zero degrees) is normally desirable. VELOKS MK3 comes with neutral camber, but it can be adjusted +/- 5 degrees if needed.
Kingpin / Wheel Axle intersect
IThe placement of the wheel axle, in relationship to the kingpin, also impacts the steering. If the wheel axles are placed in front of the kingpin axles, the caster angle effect is defeated making the steering unpredictable and unstable. I the wheel axles trail behind the kingpin, the steering may be influenced by road shock and brake steering.
Ideal position is that the axle and kingpin intersect on the same plane, like the VELOKS MK3.