For synthetine I would recommend atleast 2ml per day (or preworkout). 3ml is much better than 2ml and 4ml better than 3ml but laws of diminishing returns comes into play the higher you go. I use 2-5ml everytime though. If I am really wanting to look freaky I go with 5ml preworkout.
Often I combine syntheselen with synthetine so if I do I use 5ml syringes and inject them together. A little bit of syntheselen can go a long way for me so if I do both I will usually do 3ml synthetine and 2ml syntheselen. If I use a 3ml syringe then 2ml synthetine and 1.5ml syntheselen. Play about with dosing but again 2-5ml per day (or preworkout).
I don't use synthelamin now and syntheselen contains some vitamin b12 so I have never used them together. When I did use synthelamin I think I went with 1-2ml every 3 days. I would follow the label and see how you feel and go from there.