To the original poster, it is O2/He/N2.
Normoxic Trimix is not the basic entry level trimix course anymore. Recreational Trimix and Advanced Recreational Trimix are the entry level courses and read very much like Nitrox and Advanced Nitrox.
Hypoxic mixes are considered to not sustain life at the surface, and therefore, require a travel gas to get the diver to a higher ATA and PO2 before breathing it. Normoxic mixes would be considered safe at the surface down to their MOD.
The first part of the equation, O2, is to figure out your Target Operating Depth and choose a FO2 appropriate for that depth with a MOD acceptable for your target PO2.
The third part of the equation, N2, is to figure out your target Equivelent Narcotic Depth. This will give you an idea of how much N2, or FN2, you will want in your mix to keep your narcosis managable. Most divers tend to have a standard limit of 60, 80, or 100 fsw for END/EAD.
This leaves the second part of the equation, He. Basically, you add FO2 + FN2 and subtract from 100. This leaves you FHe. All you need to tell your fill station is the FO2/FHe, since FN2 is the balance.
As for the limits of maximum FO2 for trimix, it depends on your certifying agency, the course that you certified under, and the depth you are diving to. There are no maximum FHe, other than it would have to be equal to or less than (100 - (an FO2 appropriate for your MOD).
