It depends, the more competition you are into, the more natural you need to look. So, if you are after high difficulty keywords the best combo you need is:
-High quality domain name for the PBN (Domain authority / Page authority).
-Relevant Contextual link. If you are talking about health, the article in the PBN should cover this subject and it should be naturally. Google AI and deep learning algo is getting better and better at defining what good content is. Majestic metrics can help here, also. To optimize
-If you decide to have a link from the homepage, then keep your OBL low, which will make your investment higher as you will need more PBNs.
-Best way to optimize your PBN link cost, is to focus on specific sites covering a same general theme/niche. For instance, you could decide to go after the dermatology theme, and have one site talking about herpes, another about psoriasis etc... If you check the first google results, you notice that the CF metric helped a lot in placing that website first, ahead of webmd and mayoclinic. The same site has locked the 4th position as well thanks to a 2400 words high quality guide with an easy to naviguate layout and a css naviguation in the left. So, the more relevant you are, the more research and effort you put in, the more rewards you will get.
Overdoing it by sending several links to your money site or having a high OBL for the homepage of your PBN, because you want to ''optimize" your ROI will just get you nowhere. Especially in the highly competitive english.
French and Spanish ranking is another story and is much easier, I would compare it to ranking in Bing/Yahoo search engines.
But again, take sometime if not done already to approximately know how much each conversion/lead is worth, dont invest blindly for the sake of being on first page if you end up making peanuts per conversion.
