The normal dose of Phentermine is either 15 or 37.5. I would just use the 37.5. 15 is pretty worthless IMO unless you've had ZERO stimulants in your life prior, which lets be honest, in BB thats nothing.
I liked it when using DNP, helped curb cravings pretty well.
Vyvanse/Adderall works better IMO.
Phentermine effects wore off for me within a month.