It's easy. Just remember 1 cup flour, 1 cup liquid, 1 tsp soda, 1 egg, and 1/2 tsp salt (there's a joker in every deck), and you're in the money.
Use soft flour or make your own soft flour by using 3 parts AP to 1 part corn starch, or make it even softer by mixing the two in equal amounts.
For each cup of flour, add between 1 and 2 tsp (total) of baking soda, or a combinatioon of baking powder and soda in equal amounts. If you're holding the batter at all, go with the combo.
Add about 1/2 tsp of table salt per cup of flour.
1 part flour to 1 part liquid. Use very cold liquid. Ice water is fine, but beer and seltzer each make for a lighter crust. Beat the wets into the dries until just incorporated -- it doesn't need to be lump free.
For each cup of flour, use 1 egg. Beat the egg before adding it to the batter, using as few strokes as possible. If you want the batter very light, separate the egg, beat the yolk and add it to the batter. Then beat the whites to the soft peak stage, and fold them in.
You can make a very light tempura batter by omitting the egg altogether, and upping the liquid to 1-1/2 cups seltzer per cup of flour. Some people might think of it as being more Japanese.
If you want to a sort of Chinese take, you can add a few drops of yellow food coloring.
You may also add herbs such as parsley or shiso.
If you hold the batter, hold it cold in the refrigerator. Only use the batter cold. Dry whatever is to be fried as much as possible, and dust lightly in corn starch before dipping, so the batter will cling and not blow up too far off the food.
Just as an addition, notice that throughout BDL's post the empasis is on light. Tempora is not, repeat not, batter-dipped in the Western sense. It's very light, applied thinly (some authorities say if you can't see through the fried batter it isn't tempora), and, did I mention, light?
Personally, I use rice flour (all other relationships pretty much as BDL says), but that's must my preference. No reason not to use a soft wheat flour, as he specifies.
Cook's Illustrated says vodka is a vital ingredient, because the alcohol contributes wetness without adding to gluten, which makes the batter thick. I believe the use of cornstarch and keeping the batter cold as BDL mentioned is also meant to hinder the development of gluten. I would post the Cook's recipe but I don't know if I'm allowed.
I like to use rice flour, seltzer, soda water, or beer/sake, a touch of sesame oil (sometimes with sesame seeds or other seasonings/herbs added), and salt to taste. I add egg as necessary, but I tend to use just enough to bind, if any at all, and sometimes I will just fold in a little whipped whites without yolk. Sugar may be added, but only a slight amount if any. I don't have a direct recipe, since it always depends on the consistency I want for the specific product, and I test/play around with the batter each time I make it before I go full-on production with it, but the ratios are always close to what BDL posted.
Rice flour is the best for me. Only thing I do a bit different is add a dash of cornstarch. Reason being the minute the starch comes in contact with hot oil it sets therefore oil has a hard time penetrating to the product. I have played with flour and soda and baking powder, but always came back to rice flour.and ice water. I also add a drop or 2 of yellow egg shade for eye appeal.
If you can't make it after Bdl explanation, then forget it. The only thing I see most cooks do is, they make the batter to thick. We in the West, always have ever thing in a heavy batter. If the Tempura batter looks way to thin, its probably just right................Server with a Daikon dipping sauce....................The Best...............ChefBillyB......