Where should a TV aerial be positioned?
There are many factors that affect signal strength, some are in your control like antenna placement, but most are not (other buildings/structures, topography, location of broadcast, atmospheric conditions, etc.).  

If you are trying to find the "best" placement then you need to do a site survey.  The crude version of this is using a long temporary cable, moving the antenna around to different locations and viewing the signal strength from your receiver ("box") or TV.  Professionals will use instruments like signal meters of varying sophistication. 

One nice attribute of digital OTA is that being binary (ones and zeros), picture quality isn't affected - it either works or doesn't work (for the most part).  If you placed the antenna at your desired location and could get say 75% signal on most channels, then you could very likely use that location even if there were better ones.