I do exactly the same thing, sometimes--I approximate based on a similar dish elsewhere.
More often, if I can't measure it accurately, I just don't eat it. This was a big breakthrough for me: previous attempts at calorie counting failed in part because, when faced with something I couldn't accurately count, I would freeze. Once I'd decided to eat something, I HAD to have it, and if I couldn't count it, the whole system fell apart. Realizing I needed to prioritize "countability" up there with "taste" was a new idea.
This doesn't mean you can't eat out places that don't post nutritional information--it just means that when looking at the menu, "countability" should be one of your concerns: if both the steak and the fancy salad appeal equally, the steak is actually much, much easier to account for than an elaborate salad, so go with the steak (it may also have fewer calories. Salads--really yummy ones--can be killers). The hamburger is probably easier to approximate than the wraps. The shrimp is probably easier to approximate than the pasta. The boxed ham or turkey sandwich is easier to figure out than the chicken salad sandwich.
So if I am stuck out and feeling panicky, I pick the thing that I can figure out AND that I like. Something usually meets both requirements.