Have had to do a manual one recently, due to the ground- handling company delegating someone who was not experienced in 747 ops and started off with the wrong basic weight and index. Took about 15 mins to do and have checked by my co-pilot. The trim-sheet is more important, as the computerised flight plan (LIDO) should give you the opportunity to have a rough cross-check of ZFW, TOW and burn-off for LDW. However there is no rough guide to the trim, especially on a freighter with up to 115 tonnes of freight - that's where the head-loader earns his/her money!