Anthony,
If I understand your question properly, I would reword it as: What pilot hole diameter should I use in reaming and how does the type of reamer (chucking vs. hand) affect that? If that is a correct restatement of your question, then:
The diameter of the pilot hole is a proportion of the diameter of the finished, reamed hole size dependent on the material being reamed and whether or not you are driving it with a machine or by hand. Low-shear-strength materials such as aluminum and brass my use smaller pilot holes than high-shear-strength materials such as alloy or high carbon steels (etc.). There is also a dependency on how well made and properly sharpened your drill bits are (cheap or poorly sharpened drill bits drill a significantly oversized hole in most cases). There is a section in the SME Manufacturing Engineer's Handbook that gives formulae for such things if you are interested in a more complicated answer. But:
Machine Reaming:
In Aluminum and other "soft" (low-shear) materials, I use the general rule of thumb of: ø - .010/.015 (in inches) for holes that end up 5/16 inch or smaller; ø - .015/.025 for holes between 5/16 and 1/2; and ø - 1/32 for holes between 1/2 and 1.
In alloy steel and other "tough" (high-shear) materials, I use a general rule of thumb of: ø - .005/.010 (in inches) for holes that end up 1/4 or smaller; ø - .008/.012 for holes between 1/4 and 3/8; ø - .010/.016 for holes between 3/8 and 5/8; and ø - .015/.025 for holes between 5/8 and 1.
Hand Reaming:
Back in the dark ages when I actually did quite a bit of hand reaming (excluding tapered pin reamers), I tried to use about 1/2 the allowance I used when machine reaming. I was also (A) using my hands in the shop for 6-10 hours/day and (B) a serious high-angle ice and rock climber, so I could remove quite a bit of material merely with my wrists. I would hate to try to hand ream a ø1.000 hole today from a 63/64's pilot hole in 4130-HT steel. I don't think my wrists are up to such a task today.
Your mileage may vary...
