When you need to capture a portrait and you experience the presence of more than one light source, it can be difficult to get the perfect exposure. In this case, you will have to use spot metering on the camera. Switching to spot metering lets you differentiate the exposure you need for different light sources. No matter whether you try to figure out ambient or flash, spot metering allows you to meter depending on the exposure level you need. This is because spot metering gives you the opportunity to meter the area of your choice regardless of the exposure balance of the entire scene. It also allows you to decide about the artificial light source you are using.
To accomplish better exposure in the presence of multiple light sources;
Configure your camera to spot metering mode. Then , meter for the face of the subject
Then, meter the camera to match the output you need
You can use a handheld light meter to determine the right kind of aperture (if you shoot without a TTL). If not, you can use any aperture value and rely on the flash to meter the scene. However, if the flash is unable to meter, you should go to the manual metering.
Just remember to configure the shutter speed of your handheld light meter to the fastest possible value. Such approach allows you to get rid of the potential ambient light.