Well I do precision sheet metal for an occupation so I am used to holding fairly tight tolerances with my work, but you can get good results just keeping the dimensions of the patterns within a fraction of an inch(say within 1/32" ) measuring with a tape measure or scale. Keep in mind the shrinkage rate for aluminum which Dave Gingery teaches in his book(about 1/8" per foot f I remember right) For me to make it easier to figure the sizes of the pattern parts with a scale factor and multiply the sizes by that factor, and although you can make a shrink rule equivalent to use when you measure I just found it easier to work with the scale factor. [12.125 divided by 12=1.0104] and that is the factor you can use to figure the sizes with shrinkage included. Another thing that may help checking the dimensions is a set of calipers, but that will be more evident when you get to the metal machining etc.
I have all the patterns for the 3 machines stored in boxes, and I had thought of making a set of drawings for them all in a .dxf format so they could be CNC machined but am not sure if that would be OK with the Gingery's or if anyone would even be interested. Probably the people with a CNC would not be so interested in manual machines but for myself they do come in handy for simple machining.