aviationluver, are you only looking for a formula to be used with a calculator, or a method to quickly get a result in flight?
Formulas and calculators will obviously give near-perfect figures, the question may be, on the practical side: can you use them in flight in a dynamic environment?
As previously posted, 1% TAS (or GS depending on your needs) is a good rule of thumb, provided the aircraft maintains a rate-one turn.
Beyond 180KTAS, maintaining a rate-one turn is not an option for commercial (carrying pax) aircraft. Most flight directors limit the bank angle to 25 or 30 degrees. In these higher speeds situations, the 1% calc will not fit.
But you can find the turn radius for higher speeds based on the Mach number, or the speed in miles-per-minute:
Say M0.7 or 7 miles-per-minute (MPM), just subtract 2 and you get the radius in NM. This is really close to a formula used with a calculator, and you can use it instantly while in flight without losing focus on what you're doing (flying I guess?).
This is described in "M3: the Mile, the Mach, the Minute", mental math for aviators, with the correponding formulas.
Note that it does not take into account the time (and thus distance) travelled during the intiation of the turn, from level flight to 25 or 30 degree bank.