Hi,
I hope this will help you.
Usually there should be a table at the back of the flash to show distance,
ASA. and aperture to use.
Eg: At 10 feet, ASA 100 the F-Stop usually is F5.6 - all this is for a normal flash with GN of 30.
Base on Inverse law of light, every change of distance will change the power of light. Eg: if the distance is cut by 2 feet to 8 feet, your aperture should be f8. Vise versa if it add another 2 feet, the f stop should be f4.
Another method is using light meter. use a measuring tape and measure the distance. Start with 10 feet distance and fire the flash to test the f stop. Use every 2 feet in change of distance.
Is your flash is purely a TTL flash for the camera made - eg nikon or canon?
Is that is the case, perhaps it has no flash distance table at all.
Then you need to change to a higher model flash of your camera made( eg nikon or canon).
Hope this help you.
Regards
chunsan