I thought I would share how I finally figured out the decimal system to enter pick-up notes.
The first step is to determine the beat unit of your time signature (denominator).
Then use the popover window Shift M, enter your meter, then a comma, and the number of beat units (or portions of a beat) for your pick-up.
In 4/4, if you wanted to add a quarter note pick-up, you type: 4/4,1 because a quarter note is 1 beat unit.
4/4,.5 is an eighth note pick-up because it’s half of a beat unit
4/4,1.5 is a dotted quarter pick-up
4/4,2 is a half note pick up.
The same system works for ¾, 2/4, or any other meter with the quarter as the beat unit.
If the meter is 2/2 and you want to enter a quarter note pick up: 2/2,.5
For compound meters, the eighth note is the beat unit, so if you wanted to add an eight note pick-up, you would NOT enter 6/8,.5 like in 4/4, you would enter 6/8,1 since an eighth note is only 1 beat unit in compound meters.
9/8,3 = a dotted quarter pick-up because 3 beat units (eighth) equals a dotted quarter.
12/8,.5 = 16th note pick-up
Hope that helps.