What percent adjustment is left for this chain saw bar?
Given new information that reveals details not disclosed in the original question:

So, nothing in there about taking the bar off to adjust the chain, though it is just after the point where they describe removing and replacing the chain. Since you appear to have (from your linked parts diagram, where I found the model number so I could find a manual) an electric pole saw, not a normal chainsaw, and it evidently has a complete lack of oiling system I kept that in the picture when taking a clipping from the manual for the purpose of this question. You'll want to be dedicated about oiling the thing. The adjustment range is likely plenty, since the bar and chain are tiny by normal chainsaw standards.
End of edit from New information provided
The pin moves the bar. The movement of the bar tightens the chain. The position of the pin, diassembled, is mostly irrelevant, though it might indicate a nearly worn-out chain. The odds that you've worn out a bar (yet you are asking this question) are minscule.
When assembling the bar and chain onto the saw body, the pin is normally as far back as it needs to be to allow easy assembly. At that point, one tightens the chain, by moving the adjuster, either via a screwdriver or using one of the complex &quot;easy&quot; mechanisms that have crawled all over homeowner saws in recent years. For the screw adjustment you typically have the blade nuts snug but not tight, and adjust so the chain moves easily but does not come all the way out of the slot when you pull on the chain at the bottom of the bar. Regardless of the mechanism type, the adjustment procedure is detailed in your owner's manual, which is almost certainly available on the web if you don't have a copy you can find at home. Follow it. Mine requires me to hold the tip of the bar up while tightening the nuts after adjustment, for instance.
As noted in my comment, one way to kill a chain is that as you use the saw, the chain heats up, gets longer, and becomes slack. You need to tighten it to continue to work safely. But when you stop working for a while, you need to slack it off, or it will shrink as it cools, and having been tightened while hot it will stretch as it cools - this also stresses the sprocket/shaft more than is desirable.
A used-up or worn out BAR is assessed by the depth and condition of the slot the blade teeth ride in - that assessment has nothing to do with the chain adjustment pin. Few people who are not using a saw professionally wear out a bar.
Chains are a consumable item. You should have at least one spare, and perhaps several if you don't do your own sharpening - or even if you do. In the event of chain damage, it's usually less disruptive to swap to a sharp chain and keep working, rather than having to fix the teeth that ate a rock (for example) while in the woods.  When a chain is worn to the point that it's either past more sharpening, or can't be tightened due to wear and/or stretching, retire it.