Studding the soles of felt boots is quite easy to get right and quite easy to get wrong!
First thing to check is that the thread on the studs are not longer than the thickness of the felt - obvious one that innit?
I use 8 on the heel about 3/8" in from the edge and another 7 on the sole about 1/2" in from the edge. Don't put any under the center area of the ball of the foot as you will not be very comfortable and your boots won't last very long either!

I fish the Liffey and it varies in bottom composition and have no problems with this set up, mud on the banking is a different story!
OOPS ! I had to check and as they say a picture is worth a thousand words! This set up works for me.