Flow bench is not as useful as it might seem. Yes it helps and good bench numbers equal good hp numbers if everything else is right. But remeber the first thing they tell you when using a flow bench is its only about 1/4 the speed and pressure of a running engine. The real engine will force air thru the head much faster than a small vacum can. So yes they work but can lead to poorly ported heads if you do not understand flow to begin with.
Air flows down the center of the port. the air along the walls is moving very slowly and not really where the largest flow comes from. but air moving down the center of the port can reach and exceed the speed of sound. Your goal as a porter is make the port flow enough air to get close to the speed of sound as possible without going past the mark becasue air get tubulent after that. But until it reaches that speed it will increase in flow to the cylinder as the speed increases. Until you reach the tipping point where the air get turbulent. Since the flow bench cant actually push air with that much pressure most of the time you need to calculate the port size and shape. and when that i done it is almost always smaller than the one you have in front of you to cut.
Here is what I use to port heads. this is a little small for the job with cast iron car heads but will work. But its amazing on alum car and bike heads. If you want it perfect this or a tool like it is the only way to go.
K.SRH440, Industrial Kit with heavy duty shafting, 115 Volt
Foredom has been making precision tools for many years and a lot of professional use them. Price sucks and they dont go on sale. This would produce much better results for your money spent than a flow bench. Now both should solve a whole lot of problems. But most head porters make there own tools. Or use tools they have had for years to do certain parts of the heads.
If you want to become a master at porting best to go to the junk yard and buy about 20 heads off anything and start cutting. Eye and ear protection is very important. Your not a porter until you been to the hospital to get a burning hot peice of metal removed from your eye. Also is a really good way to remeber to where your safty glasses. One trip to the hospital and you never forget again.