For general principals, see Headers (Sideroom)
[svrfsM:N:J] - Set running-footer sideroom. Parameter M sets the fixed amount of space reserved on the left for the largest page number expected, and N for the same on the right. (When centering running footers, the two amounts are actually used only in their combined total.) The default is 8:8 (total 16). As an example, [svrfs5:5] would reserve only 5 spaces on either side of the running footer (10 total), permitting a longer footer to fit.
The optional parameter (J) may be specified to give the running footer justification. The footer is normally centered, but may be justified left with a value of "l" (as in "lemon") or right with a value of "r".