Angle Bracket on Feltmate Jig
Some of the manufactured jigs have a plate or disc that secures the gouge from the top and also orients the flute consistently with the jig (Kelton, Truegrind.) Other jigs fix the gouge from the bottom with the flute pushed up against a flat surface, also orienting the flute consistently (Ellsworth.)
The Feltmate jig uses a 1/4" or sometimes a 3/16" screw eye (ground to a rough hemisphere where it contacts the flute.) While simple and effective, this can result in the flute being tilted one way or the other in some cases, and can lead to a bias to grind one side or the other a little differently than the previous time. I press the "L" bracket down on the flute as I tighten the screw eye to insure that it is always oriented the same way, and get a very repeatable grind on the gouge wings, normally one or two passes.
The length of the leg is important, not only for the resultant grind, but also in my case to have a constant arm/pocket extension for a given bevel angle for different diameter tools (1/4, 3/8. 1/2, 5/8.) I have a jig for each, or maybe share adjacent diameters in some cases. Darrel provides a drawing for this on his web site:
http://www.aroundthewoods.com/imgssharp/sharpsketch.jpg
I doubt that 1/2" one way or the other on the leg matters much. it is more important that the your jigs be consistent.
It is also important that the height of the arm pocket that receives the leg be located correctly with respect to the grinding wheel. This is also shown in the sketch. It is well worth reading Darrell's entire article and reviewing all the pictures. Again the site is:
http://www.aroundthewoods.com/sharp.shtml
Hope this helps.
Jerry