Click here for the post on how much Polawax to use!)
The suggested rate for Polawax - and only Polawax - is 25% of the oil phase of your product. If you're using e-wax, you might need to use 1% to 2% more for stability. For BTMS-50, there are all kinds of suggestions, but I tend to use it the same way I use Polawax so I can change one for the other when I'm in the workshop and don't feel like doing a bunch of math on a plastic sheet with a grease pencil.
If you're using an emulsifier that isn't Polawax, you need to check for the suggested usage rate. For instance, I'm researching Lanette N right now and it seems like you'd use about 8% for 20% oils, which works out to 40% of the oil phase. Ritamulse SCG (aka Ecomulse and Natramulse) is generally used at 8% as well in low oil products (say 15% or lower) and high oil products (up to 30%).
An aside...If you have an oil phase of 20% and you have 8% of an ingredient in that oil phase, that 8% would make up 40% of the oil phase. (8/20 = 0.4 which equals 40%).
For the HLB system, there really are no recommendations for how much to use in a product. I start at 4% because it works out well for me, but you could go to 2% if you wanted or up to 8%.
If you are using an emulsifier that isn't Polawax, you really need to check the usage rates. Do not assume it is 25% of your oil phase. Look at the data sheets (if you can get them), go to your supplier's website or write to them, ask for help in a forum, or look at reliable recipes made with those emulsifiers and try to figure out what you need to use. (I hope that didn't sound sarcastic, but I'm finding it really hard to find reliable information about how to use these emulsifiers!)