For general principals, see Headers (Sideroom)
[svrfsM:N] -- Set running-footer sideroom. M sets the fixed amount of space reserved on the left for the largest page number expected, and N for a similar amount on the right. (Though specified separately for future compatibility reasons, the two amounts are actually used only in their total, because footers are always centered.) The default is 8:8 (total 16). E.g. [svrfs5:5] would reserve only 5 spaces on either side of the running footer (10 total), permitting a longer footer to fit.