The final ten strokes of a race are crucial; they are the "empty the tanks" strokes where you want your rowers to be able to expend every last ounce of energy without holding anything back. There is a saying, "the last stroke you take is the last stroke you can take," so a coxswain can make a real contribution to a boat if the rowers know exactly how many strokes they have left. There is a special room in hell reserved for coxswains who miscall the final ten (as well as buckets of coxswain blood on boat bay floors) and who either have to say "uh, sorry, two more," or who waste two or three strokes by leaving too much margin.
Counting the final ten strokes accurately takes practice, doing it over and over again until you know how far your boat will go in ten strokes. Some people simply are more used to using the spatial functions of their visual cortexes than others. (I am not one of those, incidentally.) The distance will differ for eights and fours; it will differ according to headwind or tailwind or current. Ultimately, it comes down to a certain boatfeel and a lot of experience, but here are some techniques you can use to speed the learning process and make it more of a science.
Most importantly, make yourself conscious of distances every time you practice a piece with your boat, in all conditions. (This has uses in situations other than the final ten.)
Test yourself not only whenever you are practicing the final thousand or final five hundred meters, (when you will actually be counting the final ten), but also when you are doing any piece. Silently test yourself as you come up to the end of, for example, a middle five hundred. Pick points during pieces when you think you are ten strokes from them, and see how right you are until you get better and better at it. If you are doing power tens, you can test yourself by picking a point out ahead of you on the water that you think your boat will get to. (Bear in mind though, that a boat going into the final ten of its sprint may have a lot more speed on it than a boat doing individual tens.) The more familiar you become with distances -- even at different ratings and stages of the race -- the more you will be able to think ahead during various stages of a race, and to anticipate how the blocks of the race will fit together.
Don't be afraid to tell your boat, "I'm going to practice calling the final ten on this one until I get it down."
Distances over water can be deceiving; appearance can vary depending on how smooth or rough the water is. Cross-correlate during practice by picking points along shore to get a feel for how far ten strokes looks against a fixed distance, and combine that with estimating water distance; the two will help reinforce each other.
Naturally, if you race on a home course, you can pick out landmarks during practice that you can use during a race, but even on an unfamiliar course, the practice you have gained estimating distance based on fixed points will be helpful.
On a buoyed racecourse, the standard interval is ten meters between buoys. An eight with decent speed and ratio covers roughly ten meters per stroke. Use your practices to figure out your particular boat's distance. A four might take eleven strokes to cover the same distance an eight covers in ten. Obviously, headwinds and tailwinds adjust this. Get a feel for what 100 meters looks like in various contexts, over water, on land, while driving, riding a bicycle, any situation you can think of until you just know at a glance, "that's one hundred meters."
In theory, if your bow is ten buoys from the finish line, that will be your last-ten-strokes call; however it's not as easy as it seems. Studies have shown that the human eye can apprehend groups of three -- or at most four -- objects at a glance; more than that requires conscious counting. Linguists have theorized that that is the reason that 3 and 4 were not only "sacred" numbers in a number of cultures, but also that it is the reason why -- in many languages -- the numbers up to three, or occasionally four, have declension endings (especially the ordinal numbers) and the higher numbers do not. (e.g. in English, "first," "second," "third" have unique endings, but beginning with "fourth," all numbers end in "-th." Latin declines "uno" "duo" and "tres" but not "quattor" or higher.)
Whatever the biological reason, during the last ten strokes of a race, there simply isn't time to count buoys, especially when the coxswain needs to be doing a lot of other things such as making calls, keeping track of other boats which may be charging, giving the building strokes for the final stroke count increase, etc.
A trick to get around this is to speed-count the final buoys as 3-3-4, which your eye can do almost instantly. It's even faster if you count them backwards from the finish line, so that the group of four is closest to you. If you are coxing a four, you might do it as 3-4-4.
None of these tricks is foolproof, but used in combination they will help you get better at it. In time, it will become almost second nature, so that you'll just know when it's right.