If you are spacing right and are able to drive off a decent flake, it is likely that the reason they are "all over the place" is due to not having a uniform edge. I mean uniform in grinding, thickness, angle and center line. Sure, the way you apply pressure can totally affect the length of a flake, but in order to get a flake pattern, you have to set up the point in a way that allows you to repeat one flake after another - a pattern. If your grinding, thickness, angles, and center line are all different along the length there is no way you can produce a series of flakes that are the same along that length of variables.
FYI, I used to do that rolling thing, but I later wound up using my legs top power off flakes. I think you can use any method that works for you and they all are probably ways to approach the same result with different mechanics.
Here's my pressure flaking process once I am through with percussion.
1. Look at the center line along the edge. Grind and pressure off anything below the center line on one edge.
2. Flip the point and on the edge you just knapped, re grind and pressure off anything below the center line. You will end up with an even edge - nothing dipping or raised.
3. At this point, the center line may be in the center. If it is more to one side than the other just make sure it is ground and take a row of flakes base to tip. If it is really in the center, pressure small flakes along the entire length straight down to move the center line up. Flip the point over and grind it so that you can knap the whole length of the edge in the below the center line set up you just created.
As a detail, when I grind, I don't just "grind." After I take off the popped down flakes to move the center line up. I take a carborundum grinder and brush it downward across the edge - this is perpendicular grinding - not lengthwise grinding. When I do that it takes off even more tiny flakes, moving the center line a little more up. Then I flip it and do the same brushing type grinding again to insure I have a really uniform edge. Of course you will then flip the point so that the center line is towards the bottom.
4. I hope this is making sense because I never even think about this. The process just repeats from here except that since flakes are spaced out, there is a flat ground spot between them. There are several ways to handle this and there are at least two really good ways to approach spacing flakes....this is getting long.
A. Spacing flakes while making straight ridges to follow - So, you take off a flake. Look at the scar of what you just did. There is the spot on the edge and a scar shaped something like a guitar pick. Flakes like to follow ridges, so you have to decide whether you will just skip down a quarter inch and take off the next flake or you want to create a straight ridge. If you are going for a straight ridge to follow between flakes, then just beside where you took off that very first flake, you will need to take off one, two or maybe even three smaller flakes until that guitar shaped flake has a straighter edge on it a quarter inch down for your next flake to follow. Jim Winn (Paleomanjim) has an excellent video on this if you don't understand what I am having extreme trouble writing.
B. Spacing flakes - Just pop off a flake, skip a quarter inch and pressure off another. In the flat spot between - called the delta, just pop two to three tiny flakes off in between them. If all was well in your edge to begin with, you can easily do this.
That's about it. You have to prepare the entire edge each time you take off flakes. There are four sides to every point and four times you must repeat everything if you want a flake pattern.
Good luck - I hope this makes sense.
