No body has mentioned mirage yet......... Where I shoot at times I am in the dirt prone, there always seems to be some mirage, on lower power settings I tend not to chase the target as much, which I shouldn't do in the first place..
If you don't have a way of calculating density altitude just use station pressure, temperature, and humidity. That is density altitude, Guestimate humidity it is a minor factor anyway.
I mainly use a chrono to find out how consistent I am reloading, consistency of a load, (es & sd) and to get a rough number to plug into my ballistic program. After that all dope is done by shooting and adjusting the program
I agree 100% with all that sterling shooter has said, and the reason for that is, I have experienced everything the OP has gone through over the last 43 years of shooting and hunting, especially with the larger caliber rifles. I can get a "little' sloppy with the pea shooters and not suffer...