It’s ideal to use a depth-setting nipple driver specific to your nipple but sometimes you can’t. Sometimes a tool doesn’t exist or isn’t available but you can still do better than eyeballing. In a pinch you can make a tool for starting your nipples an equal number of turns all around.
To make a universal nipple driver I trim a thin butted spoke and roll threads in such a way that the threads cross the butting boundary. This lets a nipple thread all the way through without bottoming out. Then I attach an inverted nipple — one with a round bottom and no screwdriver slot — leaving a bit of spoke thread protruding. How much protrudes depends on how far you want the driver to work. I use some Loctite so the nipple doesn’t move. In the following example I inserted the tool into a pin vise but you could glue it into a cork, dowel, etc.
To use the tool thread it onto the back of your nipple. Insert the loaded tool into the rim and thread onto a waiting spoke. Sideload the spoke to hold the nipple, unthread the tool and repeat. This isn’t the fastest way to work but it gets the job done and costs very little. With all your nipples preloaded the same amount, you’re off to a clean start.