I've lashed rockers multiple ways but the best for me has been to warm the engine up to operating temp, cut card board to the length of the head, pop the valve cover off, slide the card board into the space between the valve springs and the outside edge of the head, slowly back off each rocker one at a time until the lifter and rocker seat start to tick (engine running). Then tighten the rocker back down slowly until the ticking stops then give it a quarter to half a turn (depending on your preference). I prefer a quarter turn on all rockers but internet arguments usually ensues after that statement as everyone is a hotrod specialist these days on the choice of a quarter, half or full turn on the rocker nut. The cardboard keeps the oil coming from the pushrod and the rockers from splashing off onto the exhaust manifolds causing a smoke show. I've always conducted a leak down and compression test while I'm there to check the seal of the valve seats and piston rings to cover my A**. I lash rockers off the base circle of the cam only when I rebuild engines and the block is out of the vehicle as it's convenient to visually ensure the lifter is actually on the base circle of the lobe and I can jiggle the push rod to verify the rocker, push rod and cam are at zero lash for hydraulic lifters then add my specific preload turn to the rocker nut.