It depends Adam.
For Perch, I wouldn't lay on too much otherwise it could lead to deep hooking (useing magots). If there isn't too much pull on the canal, lay on as little as possible to begin with, about a couple of inches to see if your float holds position. A number 8 shot a couple of inches from the hook to begin with, then add more if needs be.
If you do start to drift, then move some shot down and lengthen the amount you are laying on by.
For Roach, Bream and Tench, (useing casters) I allways found that laying on by 10 inches was a good starting point.
When you first start, you will probably find that the Perch will be the first at your hookbait. Then you will start to catch the Roach, and then the Bream. As a rule of thumb, it it was usually reckoned that if you were catching Perch, then there were no other species in your swim. But that doesn't usually last for long as you begin to build your swim up by regular feeding.
