Exposure for bounce flash with a dedicated camera and flash combination that uses a TTL or OTF metering system is completely automatic and accurate in almost all situations, since the light is being measured inside the camera. With bounce flash, you lose about 50-percent efficiency. Check the maximum distance on your flash calculator dial. With some units, you can test for sufficient light without actually taking a picture. To use this feature, you fire the flash without taking a picture and watch the indicator light to see if there is enough light from the flash.
Most flash units have a button for firing the flash independently of the camera.
If there's not enough light, you will have to use a larger lens opening, move closer to your subject, or use a faster film. If you are using an automatic flash unit that has no tilt or swivel capability, the only way to use it for bounce is to take it off the camera and aim the whole unit at your bounce surface. The only solution is to switch the flash unit to manual. To determine exposure for bounce flash manually, first find the total distance from the flash to the bounce surface and from the bounce surface to your subject. If the flash doesn't have a calculator dial, divide the guide number by the total distance.