Camshaft shims and clearances

All the bottom end sorted have fitted the heads that I sent out to get guides, new valves,and set up the shim clearances by facing the top of the valve. The question, is there a measurement that the shim top face should be above the spring retainer as the bucket hitting the retainer will dislodge the collets and is there a minimum amount that the shim should be inside the spring retainer so it does not come out of the retainer? cheers Richard