First things first, we need to know a little about the engine. If it is an aluminum block or cast iron, the deck height and are the heads aluminum or iron. Stuff like that. Because aluminum grows way more than cast iron. An aluminum block can grow almost three times as much as a iron block. Similar with the heads. Also the more meat (deck height) the more material to grow. So when you have a solid iron block and heads- something like standard deck Merlin stuff, you can see as little as two thou difference hot to cold. An all aluminum real tall deck deal can grow as much as 15 thou. I have heard of more in certain very high heat applications, but lets stay with the real world for now. Cast iron chevy big block with aluminum heads with good fresh water flow in a boat, you will see less growth than a car or a dry drag race engine. You will probably see somewhere around 4-5 thou.
When you warm the engine up, it will not grow right away. Don't expect all the growth after two minutes of warming it up. Allowing the heat to sink in takes some time. So the time to get your valve covers off may be just right to get at the valves at optimum temperature and growth. If you want to get the thing warm, but not skin bubbling hot, just do so and set them a half to a thou tight for iron heads or a thou or two thou for aluminum. Make tripple sure they are perfect, then tighten the girdles down and run the thing.