The old tried an true for finding the hot/cold areas of your grilling area is to get a can of biscuits - oil your grates first of course or better yet - get some Q' Mat from
www.amazenproducts.com - and lay them in a pattern that is fairly close together - but not touching. And watch for over/under cooking in the process. THAT PICTURES...then you can use them after modification and repeating this process.
That does not give you actual temps. but it is a cheap and easy way to establish a base line of where the hot/cold areas are - then when you make modifications, and compare - you can see practical results.
Hope that helps!
T