If you're using a HSI then there's an easy way: Keep the HDG lubber line touching the top of the CDI. The closer the CDI moves towards the centre, the intercept angle will reduce. Eventually the intercept angle will equal zero when the CDI reaches the centre.
There are a couple of caveats though:
1. If there is a wind from the other side of the radial ie a headwind component during the intercept, then eventually the radial closure will stop when the intercept angle equals the wind correction angle. Not a big deal - you'll just have to recognise it and then increase the intercept angle a bit to get the CDI centred. But then the advantage is that once it's centred you'll already know what WCA to lay off to anticipate the drift.
2. If a tailwind component then the CDI could move quite rapidly, needing a fair amount of turn rate from you, and you will also have to anticipate the required WCA by turning a bit more as the CDI centres.