I would use an RF planning tool, like Airwave VisualRF for indoor, or Aruba Outdoor Planner for outdoor to do estimates and calculations on AP coverage and signal strengths.
Those tools can take care of obstructions, and interference as well and will give a much better indication of coverage, achievable speed, capacity.
Many of our partners provide professional site-survey and WLAN design services, which are more recommended than guessing and estimations.