ORIGINAL: the potemkin
how does droptanks figure in when calculating endurance?
It would depend on each specific plane and whether you can get information about it concerning it's range without a drop tank and then with a droptank.
For instance I have information on the following aircraft:
F6F Hellcat ............ w/out droptank: 1,090 miles ..... with droptank: 1,530 miles
F4U Corsair ........... w/out droptank: 1,015 miles ..... with droptank: 1,562 miles
P-38 Lightning ....... w/out droptank: 475 miles ........ with droptank: 2,260 miles
P-39 Airacobra ...... w/out droptank: 600 miles ........ with droptank: 1,100 miles
P-40 Warhawk ....... w/out droptank: 700 miles ........ with droptank: 1,500 miles
P-47 Thunderbolt ... w/out droptank: 700 miles ........ with droptank: 1,500 miles
P-51 Mustang ........ w/out droptank: 950 miles ........ with droptank: 2,080 miles
A6M Zero ............. w/out droptank: 1,160 miles ..... with droptank: 1,930 miles
N1K1 George ........ w/out droptank: 890 miles ....... with droptank: 1,581 miles
B5N Kate .............. w/out droptank: 634 miles ....... with droptank: 1,238 miles
D4Y2 Judy ............ w/out droptank: 909 miles ....... with droptank: 2,239 miles
B6N Jill ................. w/out droptank: 909 miles ....... with droptank: 2,142 miles
G4M Betty ............ w/out droptank: 3,041 miles ..... with droptank: 3,506 miles
Ki-27 Nate ............ w/out droptank: 390 miles ....... with droptank: 1,060 miles
Ki-45 Nick ............ w/out droptank: 1,243 miles ..... with droptank: 1,404 miles
Ki-61 Tony ........... w/out droptank: 373 miles ........ with droptank: 684 miles
Ki-84 Frank .......... w/out droptank: 1,025 miles ..... with droptank: 1,815 miles
Ki-21 Sally ........... w/out droptank: 932 miles ........ with droptank: 1,680 miles
As you can see, droptanks could more than double the distance (see the P-38 Lightning) or only add a few hundred miles to the distance.