With a monoisocentric 3-field technique for treating head-and-neck cancer, collimator rotation may be needed for the upper lateral fields to avoid cephalad-positioned shoulders or to avoid unnecessary arytenoid irradiation while maintaining more anterior coverage. For patients with unilateral lymphadenopathy, lateral oblique-opposed boost fields can be used to encompass the primary tumor and ipsilateral lymph nodes without junctioning through gross disease. When initial collimated lateral fields are also rotated with a gantry angle to produce oblique boost fields, however, the resulting matchline with a low anterior neck (LAN) boost field is no longer nondivergent. This can be corrected by manual adjustment of collimator and gantry angles for the LAN field using 3D treatment planning software. The goal of this study was to derive mathematical formulas to simplify this process. We used a transformation matrix to define formulas that could predict the appropriate modifications to the LAN boost field. Output from the formulas was (1) visually tested within 3D treatment planning software and (2) verified using a solid water head-and-neck phantom and radiographic film dosimetry to confirm that a nondivergent matchline was obtained in several clinical scenarios. When evaluated with 3D treatment planning software, the formulas accurately predicted the appropriate gantry and collimator angles of the LAN boost field for a variety of possible beam combinations. When evaluated with film dosimetry, the formulas were shown to accurately predict the appropriate gantry and collimator angles of the LAN boost field to within the +/- 2 mm/1 degrees tolerance specifications of the linear accelerator and acceptable for routine clinical use. The presented formulas are simple and geometrically precise. They predict the necessary manipulations of the LAN boost field to maintain a geometrically precise matchline, as verified by 3D treatment planning software, phantom dosimetry, and actual patient setups.