very quick sums, check arithmetic
Rules of thumb 700-800 models.
If you have 600kg of fuel remaining then you have no option but to land immediately! That amount of fuel is sufficient for good conditions and visually rejoining at 4DME.
A better situation is 1200kg for go around plus one hour’s fuel. At 1200kg/hr/eng that would be landing with 3600kg remaining.
Take a B737 with operating weight 37800kg and weight limitations of ZFW 54600kg; LDG 57400kg.
Landing at 57400kg with 3600kg fuel gives ZFW of 53800kg which is within limits.
Maximum payload is derived from ZFW-37800 = 16000kg.
Estimate 300kg in descent.
From 1200kg/hr/eng a cruise of 20mins burns 800kg.
Top-of-climb weight = ZFW+800+300+3600 = 58500kg.
Estimate 2.5% of Gross Weight is consumed in climb.
Climb fuel is therefore 1500kg.
Take-off weight is 58500+1500 = 60000kg.
Max distance thus configured say 100nm climb to FL330-340, 140nm cruise, 100nm descent from FL330-340. EGLL-EGPH is about 325nm by way of example.
If 148 pax at 76kg apiece that is 11400kg leaving a maximum of 4600kg for bags and cargo. Suppose the company limits you to 22kg per person hand and hold baggage then that is typically 14700kg for people and 1300kg cargo.
Sorry if that is all a bit rushed and very broad brush but that is what rules of thumb are for and I have tried to flesh out the fuel figures.