How is electron-impact mass spectrometry data formally reported?
Third edition (latest offline edition) of the ACS Style Guide (currently known as The ACS Guide to Scholarly Communication) has a section devoted to reporting analytical data, including mass-spectrometry and EIMS in particular [1, p. 275]:

Mass Spectrometry
MS $m/z$ (relative intensity): $238.2058~(44.8\%),$ $195.1487~(100\%),$ $153.1034 ~(21.2\%).$
GC–MS $m/z$ (% relative intensity, ion): $202~(9,~\ce{M} + 4),$ $200~(32,~\ce{M} + 2),$ $198~(23,~\ce{M+}),$ $142~(35,~\ce{M} - 2\ce{CO}),$ $321~(95,~\ce{M} - \ce{Me}),$ $415~(\ce{M+} - \ce{Cl}).$
HRMS–FAB $(m/z):$ $[\ce{M} + \ce{H}]^+$ calcd for $\ce{C21H38N4O6S},$ $475.259;$ found, $475.256.$
EIMS $(\pu{70 eV})$ $m/z:$ $\ce{M+}~420~(15),$ $241~(15),$ $201~(59),$ $135~(14),$ $69~(23).$
Abbreviations: $m/z$ is the mass-to-charge ratio, $\ce{M}$ is the molecular weight of the molecule itself, $\ce{M+}$ is the molecular ion, HRMS is high-resolution mass spec-
trometry, FAB is fast atom bombardment, and EIMS is electron-impact mass spectrometry.

The recommendations for ACS journals also align with the general recommendations given in RSC Experimental data policy — Guide to the presentation of experimental data (accessed 2021-04-22):

Mass spectrometry data
Given in the form: $m/z~183~(\ce{M+},~41\%),$ $168~(38),$ $154~(9),$ $138~(31)$ etc. The molecular ion may be specified as shown if desired. Relative intensities in parentheses (% only included once). Other assignments may be included in the form $m/z~152~(33,~\ce{M} − \ce{CH3CONH2}).$ Metastable peaks may be listed as: $\ce{M^*}~160~(189\to 174),$ $147~(176\to 161),$ etc. The type of spectrum (field desorption, electron impact, etc) should be indicated. Exact masses quoted for identification purposes should be accurate to within $\pu{5 ppm}$ (EI and CI) or $\pu{10 ppm}$ (FAB or LSIMS).

If you are a $\mathrm\LaTeX$ user, check out chemmacros package (TeX.SE) and its spectroscopy module (PDF) which assists with the input of various experimental data.
Reference

The ACS Style Guide: Effective Communication of Scientific Information, 3rd ed.; Coghill, A. M., Garson, L. R., Eds.; American Chemical Society; Oxford University Press: Washington, DC; Oxford; New York, 2006. ISBN 978-0-8412-3999-9.