I think one could write an Excel sheet for planning with the fuel. There's a document hosted at Avsim (707perf.zip) which lists some cruise and climb values for a given GW. Our CS 707 needs a bit less than charted, but the values should give a nice overview.
Now, on the data we have. Assuming that the CS plane
roughly follows the charts.
The rate of fuel burn at cruise. There's a cruise chart which allows for an overview on how many nm you get out of 1000 lbs of fuel. You can also use a second table to define the fuel flow rate per engine. So, for the cruise portion, we are ok.
The climb part is also covered, but I think we will see a more or less severe discrepancy there. Either way, I'm ok.
The descent stage isn't covered but we may assume that throttling back and pointing the nose down will cause a decreased fuel flow until we come to the point where the approach procedure starts.
On the
reserve fuel, I'd say it depends. You could add a percentage of the trip fuel or, on short trips, a fixed addition of fuel weight. With the charts and when knowing your cruise FL, you can look up how much fuel the engines need per hour. So if you wanted to add some hours extra, you can get your values.
On the problem to plan reversed, just jump in with a ZFW or your liking (after using the ACE) and then add 'just some' fuel to have a GW value to start with. Then plan ahead, with that given GW, and see if you would make it when looking at the fuel charts. If you would have too much fuel left, do it again with a lower GW. The other way around when it turns out that you are running too low on fuel.
To give some examples. You would start with a TOGW of some 210.000 lbs. Lets say 40.000 lbs are fuel. The
cruise chart recommends FL390 for your flight, which would cause every 1000lbs of fuel to give you approx 46nm of range. For the
climb up to that cruise flight level of 390, you can look up that, at a GW of 210.000 lbs, it would take 8900 lbs and 25 minutes on a standard ISA day.
When leaving out any considerations on extra fuel and the winds, you would therefore climb, use those 8900 lbs of fuel, then be at cruise altitude and have 31.100 lbs left, giving you a range of 1430nm (31.1 x 46) if you wanted to run out of fuel and then glide to an airport.
Now if your trip was only 1000nm, you would have consumed the climb fuel and the part of the cruise portion. Lets say your climb took some 150nm, means 850nm are left for cruising, which would need 850 / 46 = 18.5; 18.500 lbs of fuel. You've started with 40.000 so 12.600 lbs are still there and since you would start your descent sooner than just above the destination, the descent will save some fuel while the approach stuff then consumes a bit more.
If your trip was 2000nm, you would need more fuel of course. So you could start again and add some 20.000 lbs, means you have 60.000. The GW now is 230.000 lbs and the cruise charts recommends FL370 with 1000 lbs of fuel resulting in approx 41nm.
To climb up to FL370 on a standard ISA day, you need 9500 lbs and 25 minutes. Estimate some 150nm for the climb, leaves 1850nm for the cruise, which needs 1850 / 41 = 45.1; 45.100 lbs of fuel. You've loaded 60.000 lbs of fuel, 9500 are gone with the climb and 41.500 are needed for a cruise above the airport, so 5400 lbs are left. That's a bit thin, so do it again with some extra fuel.
Well, if you have wind data in place, you can estimate it's effect on the planning distance. I would just check the winds for a general trend. If you receive headwind, add fuel and run the calcs again. The 707perf docs give you some TAS values to play with, means a 'no wind' planning basis. Tailwind would make me (I'm a sim pilot!) leave the initial fuel as it is, unless my no-wind landing weight would be anywhere close to exceeding structural limits. But that's not the case on the examples given. If you want to land on short runways, that's a different ball game of course.
But, from the charts, landing a 247.000 lbs plane on a wet runway with the airport being at 2.000ft gives me just above 7600ft of landing distance. So any
major hub larger airport will be just fine and those 247.000 are your limit for the real plane. And the sim plane brakes way better.
To give an example on the winds. Lets cruise at FL370 with standard ISA temps. This gives you 458 KTAS. We have a trip of 1850 ahead, so, with no winds, you are above the airport in slightly over 4 hours (1850 / 458 = 4.04) . If you have a headwind of 70 kts at cruise, you GS will be 388 kts, means the 1850nm are gone in 4 hours and 45 minutes, so you need fuel for roughly one hour extra. Yes, this increases the GW and slightly decreases the nm per 1000 lbs of fuel. So extra fuel means.. extra fuel.
Now I never ran her on those 6 hour or more trips, just on shorter ones. Means that the roughness of those calcs may indeed backfire on you when it comes to either the max landing weight or the danger of running out of fuel. Maybe another 707 fan has set up a Excel sheet or something to help you.
Edited: Just to be clear, that sort of 'planning' isn't really worth to be called like that. No rw pilot would plan that (rough) way, so just take it as a sim special and a very vague concept of rule of thumb. So only look at it as the guesstimated thing it is.
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