Actually to add on to my last 2 posts. My angelus missal says on partial abstinence that meat can only be eaten with the main meal, but mhfm doesn't mention this. Which is correct?

On days of fast, only one full meal is allowed and it should not be taken before noon.

Is this a rule we must obey or just a guideline? Because on days I work I need to eat before 3pm as I do afternoon shifts.

Your answer is in the text you quoted: "If fasting ... impedes the ability to do necessary work, it does not oblige." If your job involves intense physical labor or the need to stay mentally alert for safety reasons, then you would take that into consideration.

Note also that the traditional requirement of the main meal taken after midday assumes the typical morning to early evening schedule. That said, you could still observe the stricter fast on your days off work. On days you work, you could still remind yourself of the penitential season without affecting the adequacy of your nutrient intake, say, swap out a favorite beverage for something unappealing.

The general principle is acts of genuine penance and reparation but never so far as these could cause real harm to self, or even to others if workplace safety is affected.