Rain that falls on a garden does not all arrive at the roots. Some of it never gets past the leaves. Some wets the top centimetre of soil and is gone by mid-morning. What is left is the only part that matters, and on a light day there is nothing left at all.
The working figure is about a millimetre and a half. Below it, assume the garden got nothing. Above it, count the difference.
2mm of rain is worth about half a millimetre to a root. 0.8mm is worth nothing whatsoever.
What that does to a forecast
It makes most light rain disappear. A day reported as "0.8mm" is a dry day as far as the soil is concerned, and the forecast is not wrong — it is answering a different question from the one a gardener is asking.
Across a normal August in Bristol, rain falls on about seventeen days and roughly ten of them deliver enough to be worth anything. Seven rainy days a month, every month, that a garden does not benefit from.
And it is not only about light rain
Fort William is one of the wettest inhabited places in Britain. It still crossed the watering threshold on 16 July, after a sixteen-day stretch with 0.3mm in it. Wet places have dry spells; the reputation of the place tells you nothing about the fortnight you are standing in.
Very heavy rain has the opposite problem. Past about 25mm in a day the ground cannot take it in fast enough and some of it runs off, so a downpour delivers less than its own headline figure.
What a good soak is
A nine-litre watering can emptied over one square metre is nine millimetres. That is the unit to think in, and it is why "a quick splash" and "a good soak" are different answers rather than different words for the same one.
A splash of five millimetres is most of what a pot can hold and almost nothing to a border. The same tin of water settles the containers and leaves the beds exactly as thirsty as they were — which is what an experienced gardener would tell you, and what the arithmetic says independently.
The practical test is the one everybody already knows: water until it runs out of the bottom of the pot. For ground, water long enough that a finger pushed in comes out wet at the second knuckle, not just at the tip.