Hi,
Have you completed any market or SWOT analysis as yet, if not, do it, as an in depth/objective Market/SWOT analysis will give you all the answers.

Just an rule of thumb I use, add 20% to the worst case scenario and if it doesn't work at that level don't do it. This takes the emotional padding out of the equation, which we all unknowingly add, and brings everything back to reality.

Hope this helps.