Light Cahills will be hatching through the end of June. The nymphs are clinger nymphs that spend most of their life cling to the underside of rocks or between the cracks on the bottom of the runs and riffles of the streams. These nymphs need the fast highly oxygenated water to exist and that is why their bodies are made to cling on the bottom of rocks.
When they are on the bottom of the streams, obviously they are not available as trout food, however, when it is time to hatch, they migrate to the slower moving water nearby. Like most other clinger nymphs, fishing with the imitations of the nymphs should be only when the hatch is about to commence or just prior to the time when the duns are emerging.
When you feel that the hatch is about to start or if it has just started, you may want to imitate the nymph moving from the fast water to the slower moving water nearby. If the hatch has begun, you would want to do this in the morning and early afternoon until the nymphs start to emerge. Present the heavily weighted nymph on the bottom on the edges or the seams of the fast moving waters. The nymph should be worked out of the fast water into the adjacent slower moving waters to imitate the actual migration. Also, focus on the current seams created by pockets.
Keep the split shot weights about six or seven inches above the nymph pattern. Add the weights as required to keep the nymph on the bottom. A short up-stream or up and across presentation will usually serve best. If a strike indicator is used, be sure that they do not keep the fly from reaching the bottom.