Does anyone know how far below the steerer tube ona carbon fork the compression plug is supposed to go? Is it 15mm like a star nut? Also how tight should it be in the steerer tube?

I have also had this issue where the headset bolt doesn't seem to get any tighter when screwed down. It's as if the headset bearings would never bind if you wanted them to. If it matters this is an integrated headset. Is it also safe to ride with it like this? Any help would be appreciated. Thanks.

I've never had any control over how far down it goes, if I understand your question correctly.

Not sure about the answer to your first question (how far?), but as for the second one (how tight?), as long as you are able to get the headset to a level of compression where the bearings do not bind and do not have slack (as evidenced by squeezing the front brake, moving the bike fore-aft and having no fork movement in the fore-aft plane), then you are able to adjust the headset bearings correctly. Not being able to bind the bearing is not a problem if you're able to get it tight enough to remove play.

I assume that you ask the first question because you feel that the cap bolt may bottom out in the compression plug?

i don't believe there is very much control on how far down the plug goes because all the plugs that I am aware of have a set length. but this should not be a problem because typically you will have a few spacers and the stem directly under the cap which is where the expansion plug is located.

You can, in fact control how far the plug goes into the steerer with certain plugs. The FSA plug is adjustable in this way and the headset won't tighten properly unless the plug is far enough down in the steerer.

I use a Problem Solvers compression plug. Hopefully it's similar to what you're using, but I'll explain how to go about doing it.

In order to install it, you loosen the compression bolt so that the assembly slips into the steerer easily. Then, thread the top cap onto the compression assembly just so you can barely see the threads showing. Now insert the whole thing into the steerer; it won't fall in because you've got the top cap there.

Stick a 5 mm in through the top cap and tighten the compression assembly until it's firm within the steerer. Now take a 6 mm and remove the top cap (leaving the plug inside the steerer). Install your stem; leave your steerer clamp bolts loose. Then take a 6 mm and tighten the top cap down on top of the stem until secure. This is what provides the headset preload: your headset spacers should be nice and tight after this.

Don't ride if the headset isn't properly preloaded. That is bad. If your spacers can move or if there's significant play when you hold the front brake while seated on your bike and rock the handlebars back and forth, then you need to tighten your headset properly.

Hopefully that all made sense, I can draw a diagram if need be.

The plug should go down into the steer tube as far down as the stem is. This way, the stem as something other than a carbon fiber hollow tube to clamp onto which could crush it. If it doesn't go down that far, than you can either add more spacers below the stem or cut down more tubing above it.

Correct. Don't let the compression plug determine your stem/bar height though. Many plugs have an adjustable depth as described a couple of posts above.