Plus not More that you can afford to loss.
I think this must be the measure to make investment as a newbie in any market. First you have to get all the required knowledge for the investing and the techniques one must have to not to be in loss. Then you only invest that amount in start that you can easily bear to loss. If you can afford a big to get profit and loss too, invest it.
You do it thats good because it is you own opinion but I think there is no minimum amount of investing bitcoin. it all requires time, patience, hard work according to money you invest. The lower you invest higher will it demand for working hard making better strategy and more patience. I started investing from 0.02 bitcoin when. And it took me a lot of time.