To be honest, this will take trial and (probably some) error on your part, as the dose needed does vary so much from individual to individual. Start low with your dose, see if it works, then go up if you need to depending upon your response.
Also, the purity of research chemical PGE-1 is typically much less than real pharmaceutical PGE-1 (Caverject, TriMix, alprostadil, etc.) I have used both human grade pharma (I am now prescribed TriMix) and the research chem stuff, and while the human grade stuff is consistent, the research chem stuff can vary from one company to another, depending on where they are getting the raws.
For real pharma PGE-1 (Caverject or TriMix), I know that 10 mcg will give me a suitable erection for penetration, while 20mcg will give me a rock-hard porn star erection that lasts for hours. So an effective dose can be somewhere in the middle, depending on what I have planned for the night. ;-)
With the research chem PGE-1, it will be more difficult to pin down (quite literally.) One research chem company offers 2mg vials, but 1mg of their stuff is equivalent to 20mcg of the real pharma PGE-1. Another research company who used to be a sponsor here, offered 200mcg vials and now I believe they offer 1000mcg vials. But it took 50mcg of their PGE-1 to give me a good hard erection, so a 200mcg vial only had 4 doses in it.
So this will be something you have to experiment with to find the right dosage for you, which is why most patients usually are given their first injection in the doctor's office so they can try to find the right dose together. My urologist knew I was already used to using PGE-1 on my own, so he had no problem just writing me a script for TriMix because he knew I was already experienced in using it on my own.
As far as how much bac water to reconstitute with, you don't need much as you want to keep the injection volume low, but use enough to make it easy to measure on the slin pin. If I hypothetically had a 1000mcg vial of research chem PGE-1, and knew my dose was going to be 50mcg, then I might reconstitute with 2ml of bac water, so that I would know that 10 units on a slin pin would be a 50mcg dose. But adjust the math to what is most convenient for you, the amount of diluent won't have an effect on your results.
Regarding what kind of slin pins to use, I used to use 29 gauge 1/2" pins, but long ago I switched to the 31 gauge 5/16" pins, and they are much better. My urologist prescribes the 31 gauge pins along with my TriMix, and they work great. You don't need the needle to be long, it only has to be just long enough to go through the skin and tunica into the corpus cavernosum that runs along both sides of the penis. Inject at 10 o'clock or 2 o'clock along the sides, never into the top or bottom as shown at the following link:
https://www.mskcc.org/cancer-care/patient-education/penile-injection-therapy
Boards like
www.franktalk.org are good for research and a lot of the inevitable questions that will arise (pun intended) as you experiment with PGE-1.
The first experience will be a little daunting to you, but after you get used to it, no problem at all and it becomes very easy. PGE-1 has allowed me to have a lot of great sexual experiences in life that I am glad that I have had. Let me know if I can help any further.