Good luck. The backpack and theodilite may give you the best results. I flew out of HKG Kai Tak for a few years,and on 31 depaytures back towards the cityand mountains, was reminded constantly by my City Fathers to look closely at the surrounds ( not too difficult). The charted heights did not include the odd extra storey added `off the Plan' and certainly not the`Cock Lofts', TV aerials etc. Not just HKG, have a close look at any western city ( even via Hollywood movies, and it is much the same worldwide). Think on during a limiting T/O, wet V1 failure, so you have 15Ft. clearance over the plans, not the above bits.
Add in a few fudge factors such as the fact that if it's been a hot day, your engines (inboards at least) are using air coming off tarmac you could not stand on, and the performance temp. comes from the S Screen in a nice cool spot near the tower. If you haven't been to the ME, ask someone who has operated through there.
Runway slope is another good one. For Performance +32Ft elevation departure end and same at other end = zero gradient. The fact that the middle of the runway was +160ft was too hard in those days. Most T/O acceleration uphill and all the stopping downhill, V1 abort: tears before bedtime. Cannot recall a better one than Bristol UK, although the Rwy profile at Leeds UK would take a lot of computing power.
Finally TOW:asssumptions that the A/C is at loadsheet wt. I of course do not load up my cabin baggage with evertyhing heavy to avoid the increasingly popular and enforced excess baggage charges. So 3Kg is a joke. 75Kg/M/65F also , even in Asia.
On a freighter I once flew, gear sensors to provide Actual ZFW were notoriously `U/S or giving false readings'. Of course declared pallet weights were trustworthy. Actual TOW is difficult to ascertain. Strange that
in these days of unlimited computer power the inability to work out Actual TOW, etc seems difficult to believe . Tyre pessures, Strut pressures available before. However as one of the City Fathers then said, about my whinge on the U/S freighter weight/balance system.. `Some things are Commercially Unacceptable' .`Would you enable a system that cam up with answer `Overweight for proposed Departure" most of the time?'.