Hey man, I'm pretty much in the same boat as you. I had a hernia surgery when I was 16 on my left teste. The doctors messed up somehow and about 5 years later the teste went up almost inside. Another few years the teste stopped producing the needed amount of T that I needed. I would guess that my T levels were around 300 before the teste stopped producing.
I assumed (correctly) that when I went to my primary care Dr that I had low T levels. I had some pretty classic symptoms... The top ones were terrible mood, very low libido (I would go flat halfway during sex), and low to no muscle gain.
One I finally got tested I ranged from 190 - 210. Now, every body is different for needs of T. In my case I think 300 was just Ok, but 400 feels much better. I am going to talk to my doc some more about possibly raising, but I'm already injecting 150 ml a week.
Now, I've heard different stories on the next bit I'm about to tell you. First, docs look to see if you have any symptoms of low T (specifically libido and muscle gain, partly mood). If you are feeling good in those departments they likely will not prescribe to you since the level of T in your body is working regardless of the reading. I've been told in about 20% of T replacement the body will stop producing it's own T and the injections take over all the work. As well, if you only have one teste, you should expect it to do all the work with your brain to produce T. Even if it's at a lower amount. Most docs don't want to prescribe if there are no symptoms due to this chance. Injecting T can put you at higher risks for certain cancers and other issues. Obviously it's better to have the T injection than to be very low (100 level) since you can develop even worse conditions than the risk of cancer.
I've pretty much assumed that my body has stopped producing most of the T, as my levels are 360-380 at the moment with 150 ml per week. My libido and mood are much better. However my muscle gain is still very slow. I've been going to the gym for 2.5 straight years, on T for 1 year. Most arm weights I'm really low at 1/3 - 1/2 body weight). Leg is about 1 - 1.75x body weight. This isn't that great for how much effort I've put in.
TL;dr: docs choice depends on your symptoms