To work out your flight level best trick I have learned is to take the total distance of your flight ( as per your flight plan) then decide that by 7.
To work out your Top of Descent ( roughly) take your flight level and devide by 3 to get your distance to go when you start to desend.
example :
- CALULATE FLIGHT LEVEL
WMKK-WSSS TOTAL DISTANCE = 200 nm
200/7= 28.57= FL290
- CALULATE TOD
FL290
290/3 = 96.6
Therefore start your descend when 97 nm to go!