3 degree GP Top of Descent calculation:
Flight Level / 10 x 3 + wind effect + deceleration segment
- Wind effect = add 1 nm per 10 kts tailwind, reduce 1 nm per 10 kts headwind.
- Deceleration without speedbrakes = 1 nm per 10 kts in level flight / 2 nm per 10 kts during 1000 ft/min descent.
- Deceleration with speedbrakes = 0.5 nm per 10 kts in level flight and 1 nm per 10 kts in 1000 ft/min descent.
Deceleration from whatever speed you are flying above FL100 to 250 kts is already included. Deceleration from 250 kts for beginning of the approach needs to be corrected for, the rest of the deceleration is easily performed by lowering the gear and flaps on approach. Its common to intercept the glideslope at 160 kts so only need to take into account deceleration from 250 kts below FL100 to 160 kts or so.
Example:
FL300
Airport elevation 0ft
Average 20 kts tail wind
Deceleration from 250 to 160 for approach in descent:
300 / 10 = 30 x 3 = 90 nm
Wind = 20 kts / 10 = 2 nm
Deceleration = 90 kts / 10 x 2 = 18 nm
Total = 90 + 2 + 18 = 110 nm
Job done, no calculator needed 
If you want to declutter even further from the above and take deceleration from 250 kts to 160 kts as a standard just add 10 nm in level flight or 20 kts in descent for deceleration, its not an exact science anyway, so:
FL / 10 x 3 + wind effect + 10 or 20 nm.
Works in the real world, been using it everyday and it is fairly accurate. Looking at that calculator it seems like it is using those exact same rules, so no need to open an internet browser while in the sim 
.