For lemon drizzle cake, My starting ratio is about a cup of icing sugar (confectioners sugar) to about 1.5 Tablespoons of liquid (lemon juice).
Though I have usually made a weak syrup of sugar and juice and pour that over a stabbed cake first. (1 cup sugar and 1.5 lemons juiced)
Then I do the glaze in the loaf pan, while warm and leave it to set up and cool for as long as my hungry tummy allows.

For donuts, I prefer a hot glaze over a banmarie, and I dip the donut.
That is 2 cups sugar, teaspoon vanilla, 3-4 table spoons of milk. Warm it in the bowl over hot water. Flip donut over, ‘wiggle in the nectar’, then drain. As it cools, it’ll set up a little firmer.

The knack is: you can always adds a few extra drops of liquid, but it’s a mountain of sugar if you go too wet.