From memory when I flew the 146 I used (FOB(in kg)- 900)/10 for a rough DP PNR before departure. After TOPC the calcs as per Timmmeee allow you to fine tune it with actual cruise outbound sgr, the cruise back sgr for 10000' will only ever be approximate because you will NEVER know the exact winds at that level. If you buffer it by say .25kg/gnm you will be safe enough...or work out a sgr into what you consider 80% probability of worst case headwinds...say -30kts or whatever is reasonable. If you get down and find only -10kts or a tailwind you will simply arrive with a little extra for mum and the kids...so what, the gauges aren't THAT accurate.
How much fuel will be used in the 180? Not much at flight idle on descent.
The DP case is the limiting case....3 eng sgr in the puff jet is better than 4 eng.
Chuck.