I want to set the page margin to 18 cm and the staff length to 17 cm, as indicated by the orange line in the picture.
However, when I add a divisi, the letters ‘I’, ‘II’, and ‘I+II’ cause the spacing to become uneven and prevent a consistent 17 cm. How can I solve this? Is there a way to do this without creating a separate template?
Staff labels will always automatically go inside page margins. Page margins essentially set the maximum area inside which music can appear.
Longer staff labels need more horizontal space than shorter staff labels. Dorico adjusts this on a per-system basis, according to the needs of staff labels on each system.
Your choices are:
-
Increase the minimum indent for staves with staff labels so that all systems have the same indent that’s needed for the widest staff labels in your project
-
Change the arrangement of longer staff labels so they’re not as wide, e.g. put a line break in
For example
I
+
II
instead of
I+II
Is it possible for the line length of I and I+II to be the same if a template is provided separately?
You can make the musical width of systems exactly the same for all systems, by increasing the margin I linked to in my previous reply, to be sufficiently large that all systems have the same left indent.
This means all systems will be the same width, although that will be as narrow as required for “I+II”, rather than as wide as just “I”.
No extra template required. You also can’t change the page margins within a layout; those are fixed for the whole layout.
Then, is there no way to make the staff length of I and I+II the same within a single file?
I’ve just explained how that’s possible 
it just requires all systems to be narrower, rather than forcing I+II systems to be wider.
Is there a way to calculate how much to move part I to the right when setting margins based on I + II?