It might be more fruitful for you to post some info regarding what you normally use, how much, and how frequently so that people can give you some advice about how to take what you have. Fent is very potent and it just might keep you from OD'ing to take some time and post some more info about your history and your intent. There are converters online that will give you an approximation of how many milligrams of oxy is equivalent to 50ug of fent but I would not go by those alone when using fent and ESPECIALLY if it is your first time with it or you are IV'ing it or both.
Also not to be nit-picky but Capt Heroin said that 50mg would not feel like much for the opiate tolerant individual - he certainly meant 50 MICROgrams. He knows that and probably everyone reading this but in the spirit of harm reduction I thought I'd mention it. I'd be glad to give you some recommendations if you can just provide some more info, but either way good luck, stay safe, and have fun!