So here's my rule of thumb for summer time LHT/HHT settings... ( OK, well mostly for HHT ).
I have found that on a reasonably nice day ( 70F, not to windy... or better ). My PG500 will reach 170F with a HHT = 15... 250F with HHT = 25 , and I've hit 650F with HHT = 55.
Oh, the rule of thumb... Multiply HHT * 10 and that will be very close to how hot the PG500 can reach in Fahrenheit.
So... HHT = 15, the PG500 should easily hit 150F. however I can tell you from experience on a hot day, running HHT = 15 , sometimes you can't really get bellow 200F depending on the pellets your using.
HHT = 25 , should get you up to at least 250F... HHT = 35 , should get you up to 350F pretty easily. etc....
Winter, all bets are off... sometimes I have to run HHT = 50 to get up to 250F on a cold winter day, and wind will only make it harder... but basically I can't give you rules of thumb for the settings in winter.
Another rule of thumb I use... not really related to HHT or LHT... Zone 3 will be roughly 20% hotter then zone 4... Zone 4 should be pretty close to the grills set temperature, zone 3 will be roughly 20% above the set temperature.
In general for summer time cooking , I don't run HHT any higher then I need to... that is, if I can hit 250F with HHT = 25, I rarely set HHT any higher. The only real exceptions are... if I expect it to get colder, or for the wind to get strong and temperatures are not at least 72F. ( again, I'm not talking winter, winter gets tricky ). For the most part, unless your trying to heat the grill up hot for searing, there's never any really good reason to use HHT > 60... If your cooking bellow 400F, really not much reason to set HHT > 40.
About the only thing I'll say about winter cooking... you often need to at least double those summer time rules of thumb. Things are probably a lot different on a PG1000 for winter, but not likely that much different in summer.