Some tips.
Cloud Base - If you can see the horizon clearly, the base is above your altitude. If the cloud base is just touching the horizon,
the base is at your altitude. If the cloud base obscures the horizon then the cloud is below your altitude.
So if you really must measure the cloud base then climb until the base is just touching the horizon. Then descend. Since cloud base is often not uniform pay attention ahead and always maintain a clear horizon.
Can't see an horizon due rain or other poor vis? What are you doing there?
If you choose to Scud Run then remember that you are better of to have your "wheels in the trees" than to have your fin in the cloud. That way you can see what lies ahead.
Horizontal distance - Very arbitary - The "what feels good" comment made by other posters is correct.
Consider your options - Dont wait for bad weather to come along - When flying VFR you should continously consider your options.
The proper order of consideration is
- Continue - As planned to destination
- Detour - Track to destination via another route
- Hold - Wait for the wx to improve- Have you fuel/Daylight for this?
- Return to starting point or Divert to a prepared landing ground
- Land off airport - A good landing on a bad surface is more survivable than a bad landingon the best surface.