Finasteride dissolves in alcohol, which also preserves it from damage (alcohol is a well known preserver). For that reason, adding finasteride pills to minoxidil is a good idea, at least with regards of keeping finasteride "in good shape". Furthermore, adding it to minoxidil will enable you to hit "two rabits with one shot", since you have to apply minoxidil anyway.
I do that and so far cannot say/prove any effects of it, but it doesn't bother me.
About the mixture - nobody knows which exactly is the proper one. In fact - heck, nobody knows which is the proper dose of finasteride taken internally :dunno: anyway. That is why you have to decide yourself. I have added 3 mg finasteride in a 65ml minoxidil bottle (Dr. Lee's) and I apply 2 ml minoxidil daily. That makes it around 0.1mg finasteride topically daily.
Good luck.