Im totally agree with the 15-20mg to start with and another 5mg isnt really that much. I would say do what you feel comfortable doing. The concensus seems to be 15-20mg. So I would deff go with that dose to start with. If your not happy with the results do a bit more at your next dose/shot. You have enough to test the waters with for your first shot. Considering everything you have said about your use, tolerance and nausea I would deff start with 15-20mg for your 1st shot. I also understand you want to experience the rush. I get that. But you really need to stay safe. Theres no doubt in my mind that at 15mg you will feel the rush!! Then next time you will know if you can or want to do more. Its not like other opiets in the rush department. If you wait at the very least an hour you will feel the same pins and needles rush the second time. Thats why I like morphine so much! I know that each time I do a shot I will have my pins and needles rush.
Let us know how things went and what mg you did!

P.S. Have you checked out the morphine mega thread? Theres alot of great info in there!!!!