I see where you are coming from, wanting to protect your HPTA. At your age, your body is producing, what will be as close to a perfect amount of hormone, that you'll ever see. No sense in messin that up.
In general with GH, the older guys are more likely to see benefits because their natural levels are lower. A guy that is 45 is going to see much better results from a few IU's ED, than someone like yourself. Using a replacement dose of 2-3iu ED will help some with fat loss as well as improve skin and hair. You may not see a big difference. If you goal is to cut, you would probably get more results using a fat burner like clen.
In order to make big gains using GH, you need to run high doses (5 to 10iu ED) with test and insulin. You will not gain 15 lbs using GH alone, especially if you only have 1 kit. For big muscle gains, GH is best for guys that have already done a lot of cycles, and are no longer responding as well to steroids alone. At this point, it'll push you back into making new gains. Without gear, I wouldn't waste my time/money, running high doses of growth.
If you already bought your GH, I would suggest you take 2iu ED for 5 months and couple it with your cutting diet.
As for mixing, its just math. If you add 1ml bac water to 10iu's, each 1/10ml is 1IU GH