Millsusaf is correct. ATF doesn't use percentages to determine the amount of 'completeness'. The percentages are 'marketing' hype. One companies 80% can be different from another's. All ATF does is determine if the hunk of aluminum is considered a firearm or not.
At one time the basis for determining whether a block of aluminum was considered a firearm was that if it took the average person less than 8 hrs to convert the block so that it could fire a single round, it was considered a firearm. Appearance/finish does not enter into this 8 hr determination, nor does the gun have to operate 100% or as designed, it only has to fire a single shot. If it took the average person more than 8 hrs, it was still considered a block of metal.
I don't know if ATF still uses this rule, as they don't seem to want to publish what they use to make these determinations. But, as far as AR15 are concerned, they seem to concentrate on 3 areas of completion....the magwell, the fire control area, and the buffer tube tower/hole. If no fire control holes are drilled, any two of these items can be completed and it will not be considered a firearm. This is why you'll see various combinations of completion of these areas, which usually vary by manufacturer. It depends on what the manufacturer sees as the most marketable solution.