No. of degrees to turn divided by 3 = seconds to turn @ rate 1.
Use anything with a compass rose on it to help figure out how many degrees to turn. It's easier to do with a 'DI' style compass than the standard wet compass restricted view.
A DI style rose also helps orient WRT to the shortest direction of turn.
Count of divisions helps, as suggested above. If you can see a compass rose then you can shortcut the counting of divisions if you can recognise when there's an easily divisible combination of divisions eg 360 --> 090 is of course 90 deg to turn and easily divisible by 3. Similarly if you're starting from, say, a couple of divisions before the 360 mark then the same number of divisions prior to 090 is also 90 deg.
I prefer to start timing as I commence the roll to turn. At the end of the timing I start the roll to wings level. I try to keep the roll-in & roll-out at approx the same rate. The slight extra bit of turn - while still with an amount of AoB at the end of the timing - compensates to some amount for the slightly slower rate of turn at the beginning prior to achieving a Rate 1 angle of bank.