Pineteam that is the US definition for declaring low fuel. In ICAO you shall declare an emergency when your useable fuel on landing is calculated as less than FRF.
In your scenario any delay should not mean you are into your final reserve fuel. It could mean that you are using your alternate fuel.
Minimum planned and calculated en route fuel for arrival IS FRF plus ALT. However, it is perfectly acceptable and legal to continue to destination if your calculated fuel is less than FRF plus ALT.