Calculating the maximum altitude of a drone
In the US, drones are required to stay in visual range.  How far away can you see?
There is also a free AOPA app that you could look into to help keep you out of trouble (or form hurting me as I fly around in my airplane at 145 MPH trying to keep an eye out for smaller things like birds and drones):

B4UFLY is an easy-to-use smartphone app that helps unmanned aircraft operators determine whether there are any restrictions or requirements in effect at the location where they want to fly.
Key features of the B4UFLY app include:
clear &quot;status&quot; indicator that immediately informs the operator about the current or planned location. For example, it shows flying in the Special Flight Rules Area around Washington, D.C. is prohibited.
Information on the parameters that drive the status indicator
A &quot;Planner Mode&quot; for future flights in different locations
Informative, interactive maps with filtering options
Links to other FAA UAS resources and regulatory information