Leadbeater's (and similar BML-type) diets are formulated homemade nectar mixes that make up roughly half of a captive sugar glider’s diet, standing in for the sap and nectar they eat in the wild.
A glider’s natural sap-and-insect diet is hard to replicate, and getting it wrong causes disease, so a vet experienced with sugar gliders should set the exact recipe, portions and supplement schedule.
The rest of the diet is commercial glider pellets, gut-loaded insects for protein, and only a small amount of fruit and vegetables.