No. You’d need to effectively reverse engineer the way that Dorico comes to those percentages, then influence them.
For horizontal spacing, that’s the values defined in Layout Options > Note Spacing (and Engrave > Note Spacing Change), plus the width of actual objects on the staves, plus all of the values in Engraving Options > Spacing Gaps.
And then for vertical spacing, it’s basically the vertical height of the staves themselves plus the values defined in Layout Options > Vertical Spacing (ideal gaps and minimum gaps) plus anything that adds height. For instance text items and dynamics have minimum spaces above/below as defined at Engraving Options > Text and Engraving Options > Dynamics. And then consider that if you have groups of dynamics, or rows of lyrics, or lengthy pedal lines (or just lines in general), they may need to budge out of the way because the staff above has a low note at one end of the system, and if it so happens that the staff below has a high note at the opposite end of the system then you’re going to get a much wider staff to staff gap (for the whole system).
It’s not quite that simple. For instance, if the staff to staff gap is defined in Layout Options as 7 spaces, and there’s a dynamic that is itself 2.5 spaces tall (plus as per Engraving Options > Dynamics it requires 2.5 spaces above) Dorico’s not going to calculate 7+2.5+2.5, it’ll just say 7>5 so there’s room for the dynamic without needing to create extra height. But then if you add into the equation a note whose stem may protrude above or below the staff, or a note that’s on a ledger line, you’re going to end up with a staff to staff gap that is necessarily more than the 7 spaces defined as the staff to staff gap in Layout Options.
As to what the value is a percentage of, that’s left margin to right margin in horizontal terms, and top (music frame) margin to bottom (music frame) margin, minus the defined padding, in vertical terms.
Then you need to add in justification: vertically, by default the gaps between staves and/or systems will grow to fit the available space, without actually altering the indicative percentage. Horizontally, assuming metered music, Dorico won’t automatically split bars across systems, so typically you’ll end up with looser horizontal spacing than is defined in Layout Options > Note Spacing, and again, this won’t affect the horizontal fullness percentage.
I’ve probably missed some values in here somewhere but hopefully it sets you on the right path.
Oh, and by way of, er, trivia? Dorico has a tendency to under-fill the last 2-3 systems of each flow, and even though lyrics are taken into account for casting off, they don’t seem to count towards to horizontal percentages in a meaningful way.
