Acceptability is in the eye of the beholder. You cannot get this "right" for two reasons:
- Different scientific communities have conflicting opinions about about the credibility of various publication venues.
- Even "good" venues publish rubbish papers.
- Some important papers are published as white papers or technical reports, and will not appear on any list of journals.
To my eyes, the real question is not about what the "right" value is, but about how the boundary that you draw will affect the conclusions of your systematic review.
For example, if you are attempting to perform a meta-analysis on the data within other data sets, then you just need a wide enough scope to be sure to get good statistical validity. Therefore, if your topic is well-studied, you can probably restrict yourself to only those publications listed in some field-appropriate major database, e.g., PubMed for biomedical literature, DBLP for computer science. It doesn't really matter which one, because you're not actually going for comprehensiveness, just for sufficient sampling, and it's more important to get well-curated data than all data. Moreover, the bad publications in the dataset are expected to be drowned out be the good ones in your data processing.
On the other hand, if you are attempting to summarize all of the credible thinking regarding a topic, then you would want to set a much broader criteria, e.g., any journal or conference with at least 5 years of publication history and not on Beale's list. In this case, you can be so generous because you are going to be using a lot more personal discretion in deciding how much weight to give each paper and interesting thinking may turn up in obscure places.
These are the two main cases that I typically see for systematic review; for other cases, you may need to adjust or pick other strategies. In all cases, however, the guiding principle is a) there is no "right" answer, and b) your choice should be driven by the effects it will have on your review.