However, what I didn’t know was that the boldenone acetate would turn the entire thing into a far-too-painful injection.
There are many other examples of how a combination could go wrong.
So as the first thing, a mixture really should be only of materials familiar to you, and with ratios that have proven themselves suitable to you and your particular purpose.
Secondly, it’s undesirable to reach an overly high total concentration. Generally I think 250 mg/mL is a reasonable maximum, but pushing to 300 is of course known and can still be reasonable. But beyond that, I really would say no. Still higher total concentrations are very likely to cause injection site pain.
Basically, you can have good results with any combination that, as individual materials, you’ve found to be good and which stays within the above total concentration.
The most efficient use is where one or more of the materials is ordinarily at low concentration. For example, testosterone propionate is ordinarily at no more than 100 mg/mL, but a still lower concentration than this may be desired for reason of being less painful. Testosterone acetate, likewise, is typically at only 100 mg/mL concentration. If wishing a more concentrated, short-acting testosterone combination, testosterone propionate plus testosterone acetate is a very efficient combination, allowing 150-200 mg/mL total.
A pointless use is combining different esters of the same steroid where there’s no need for increase in attainable concentration. For example, there is no point in combining testosterone enanthate and testosterone cypionate in the same injection.
Sustanon might appear a counterexample to that, and in terms of medical practice it is. Its combination of esters allows having both some prompt benefit and some benefit for the entire duration between office visits. Neither a short-acting ester alone nor a long-acting ester alone could do this. However for self-use, more frequent injection such as two or three times per week, combined with frontloading, provides prompt and continued benefit without requiring any combination of esters.