For a long time, I thought that "cut the tip back for some feet, then add a loop" was pretty much the whole story for converting a spey line to cast sinking tip. Recently I've found that my 6/7 Airflo Delta will cast a modest weight sink tip or floating tip as is. The reason is that Deltas have a thicker tip than some other brand lines. And that's why we cut lines back: not to shorten them, but to make the looped tip end thicker, more able to transmit the weight of a sinking tip. Not everybody has a micrometer or electronic scale lying around, but if you're into spey, you need these basic tools. For a start, it's likely that a Delta will handle sink tips of moderate weight, say under 100 grains. My Delta's tip is .046". Tips in the mid-fifties can handle most attachments, and tips above .065" handle those "logging chain" sinktips, if and when they're needed.