When creating custom SimBrief Profiles, the only control we have over altitude is a "Service Ceiling" value, which is correctly set to 35,000ft, and a "Cruise Offset" value, which is intentionally set at P0000. The former controls what the aircraft's maximum altitude is, and the latter applies a global offset to that (for example, if we set a Cruise Offset of M2000 to the current profiles, then the maximum possible cruise altitude would be 33,000ft, even on 5-hour flights).
Using a combination of the default SimBrief profile for the aircraft, as well as the values in the custom SimBrief Profiles, SimBrief then calculates what it believes is the ideal cruise altitude. We have no control over what cruise altitude SimBrief generates, other than what the maximum cruise altitude can be.
The OPT/MAX altitudes in the CDU will always be a more accurate indication of the aircraft's optimum and maximum cruise altitudes, and if a cruise altitude generated by SimBrief is greater than those values, the SimBrief OFP can always be regenerated after manually lowering the cruise altitude.
Mark - Just Flight