SPECS CAN LIE.
To cut a dovetail that fits, you have to measure the sight--even the mass produced ones. Small variations can occur in the manufacture of sights. This can result in a poorly fitting sight even in a dovetail cut to "spec." You also need to measure your cutters for the same reasons, especially if you've reground them.
Also, halfway across a hard slide is not the place to discover a high speed steel cutter was a poor choice. I use Brownells' carbide dovetail cutters exclusively, both for tool life and to easily get me through the toughest steels. Excellent carbide tools are also available from Pacific Tool & Gauge and Evolution Armory. When cutting a dovetail in a slide, the old saying "measure twice, cut once" has never been more important. Improperly positioning a dovetail, or cutting too deep, or too shallow can be disastrous. Sometimes the only fix is to make a custom sight to fit the mistake!
When you're ready to cut, it's not a race--this is an operation where speed kills (cutters, and sometimes slides). Turn slow, feed slow and keep the cutter and the slide as cool as possible. Whatever method you use--cutting oil, mist or flood coolant, don't be skimpy. It'll pay off in better cut finish and tool life. Always pre-cut your dovetails, too. Asking a dovetail cutter to plow through a hard slide cutting on a full 180-degree path is begging for an overheated and worn (or broken) cutter, a work-hardened slide and poor cut finish. I use a carbide roughing end mill to pre-cut the dovetail, leaving only the sides and 0.005" on the bottom for the dovetail cutter to clean up. It pays big benefits in tool life and cut quality.