I keep waffling on this issue. When I loose fill, I literally just let the tobacco fall in gently until it's slightly over the rim, then I VERY gently push the excess down to about even with the rim. This, in my head, fights the issue you're concerned with; namely that there won't be enough tobacco to keep an even burn. And, it seems to work for me.
The waffling comes in when I sometimes think it's better to Frank method it. I do Frank with Latakia heavy blends; theoretically the latakia is so much cooler with the burn that having it all super packed doesn't seem to overheat the pipe. But all this is just variable A in the bigger equation of a pleasant and flavorful smoke.
Draw rate is the another major factor, one which I try to keep consistent at this stage. A super slow and gentle draw is terribly important, from my perspective.
THen you get to the other variables like moisture content, which pipe you're using, lighting, etc. The grand alchemy that makes up our pursuit. In experimenting with various process options, I try to only shift one variable at a time. While imperfect, it does at the very least give the illusion that I know what I'm doing. :lol: