If your actual click speed is 1.8 clicks per second, instead of 2.0 clicks for the first spell, then the second spell will also be off, if based off of 2 clicks per second. Or the same, if your click speed is 2.4 per second while the macro was built based off of 2 per second. Click speed per second will be consistent good, or too fast or too slow, when compared with each of the macros.

You don't need to adjust for a cast time, as it is a priority sequence. While Steady Shot is firing off, nothing else can. But as soon as Steady has finished, the first ability which is ready (according to our priority basis) will fire off.

I determine the number of commas to add to a click sequence the same way as you do. Sometimes I tweak it, for 1 less commas or 1 extra commas. But usually stick with 2 clicks per second, times desired interval between castings/use of the ability.