How are frames in 59.94 drop-frame timecode dropped?
Apparently, the frames are dropped at the same cadence, but with double frequency.

From FFmpeg's timecode drop function,

if (fps == 30) {
    drop_frames = 2;
    frames_per_10mins = 17982;
} else if (fps == 60) {
    drop_frames = 4;
    frames_per_10mins = 35964;
} 
....
d = framenum / frames_per_10mins;
m = framenum % frames_per_10mins;

return framenum + 9*drop_frames*d + drop_frames*((m - drop_frames)/(frames_per_10mins/10));


and the string is

ff = framenum % fps;
ss = framenum / fps      % 60;
mm = framenum / (fps*60) % 60;
hh = framenum / (fps*3600) % 24;