Not in hitting the sides, to actually raise up the jig to achieve his geometry. If it is low, the wing angle is not enough.
http://www2.woodcraft.com/pdf/77B61.PDF
It has been a long time since I used it, but that was my understanding at the time. Some may have to do with how the Oneway setup is installed, and as an FYI, if you are going to use a Stuart Batty platform, move the Oneway back about 25mm
I am familiar with the Woodcraft instructions that show the drawings made by David Ellsworth and I think that a lot of woodturners have read more into what the holding fixture does than is actually the case. The two dimensions that are given: four inches down from the center of the grinding wheel and seven inches horizontally from the face of the wheel are strictly for setting the angle of the grind at a nominal 60° at the nose without the need for an angle measuring gauge. Once the angle has been set, the shape of the grind on the wings is primarily determined by the length of the pivot arm and the protrusion of the gouge tip from the jig.
Here is a real world example: let's assume that we have a brand new Norton SG grinding wheel (OK, maybe this is more like dream world) that has an 8" diameter. See Figure 1 for the set up as David Ellsworth describes it.
The four and seven inch dimensions place the pivot point point at approximately 11
¾" from the center of the grinding wheel. That is the critical dimension to get the desired nose angle of 60°. See the Figure 2 depicting this right triangle relationship -- one side is 4" long and the other 11" (7" offset + 4" radius = 11"). The resulting hypotenuse is 11 11/16". The thickness of the pivot arm increases this slightly so I say approximately 11
¾" as being close enough for all practical purposes.
Something tells me that, David, being a clever person knew it would be easier to follow his instructions if connected to things that we normally relate to -- like vertical and horizontal. So, this was a convenient way of locating the pivot point at the right distance.
But, given that the wheel is round, we do not necessarily need to have the 4" side of the triangle vertical nor the 11" side horizontal if something else is more convenient. The important thing is to have the pivot point located at the right distance so that the gouge will have a 60° bevel angle on its nose. From there, the size and shape of the jig takes care of the right presentation angle along the wings of the grind. And, we all know the rest ... it is up to us to not grind away too much metal and mess up the shape.
Suppose that instead of being level, the 11" leg of the triangle sloped down some arbitrary angle of about 16.5° and the once vertical 4" leg correspondingly moved so that we still have a right triangle with a hypotenuse of about 11
¾". Would that make any difference? See Figure 3 below that illustrates this.
This "something else" that I mentioned could be a Wolverine fixture. On my grinder its height is approximately as depicted in Figure 3 above. On another grinder, the height would be slightly different. The distance between the positioning arm and the wheel axis does not matter so long as the arm can be adjusted to position the pivot point the right distance from the wheel center. Even more to the point is that the distance from the pivot to wheels center is just an artifice for setting the correct bevel angle on the nose of the gouge. Now since we realize that, we can dispense with measuring distances and simply set the arm length to give us the desired bevel angle of 60° or whatever we desire if we prefer something a bit different than that.
BTW, the dimensions given by DE are not sacred. Fabrication tolerances makes each jig slightly different which means that the nose angle will be different for each jig. Additionally, as the diameter of the grinding wheel diminishes, the angle will slightly change even if the 7" dimension is rigorously maintained. It is nothing really significant, but for anyone who is a stickler for the angle being "just-so", the best approach is to measure the bevel angle directly and set the length of the slide arm accordingly.