Pastiche, using the Y & M filters with colour negatives is correct and means your negative appears to have been developed correctly.
If you add a third filter, the C filter, you are only adding neutral density and this requires more exposure to get a correct density.
If you add 30 units of all three filters, you have added 1 stop of neutral density, this is a handy feature in that it enables you to keep enlarging times constant, which in some cases can ensure there are no colour shifts with different times.
If you did this your filters would look like this:-
C 30
M 80
Y 90
And you would have to expose for one more stop of time, which in this case, would be 30 seconds.
There is one feature of colour negative enlarging not known by a lot of early colour printers like yourself, the third colour!
When you adjust your Yellow & Magenta filtration to get correct colour you are inadvertently (usually) adjusting the third colour Cyan.
The third colour filter is not used, instead density adjustments make the print more Red or Cyan.
Adding Density (time) will make the print more Red, reducing density, will make the print more Cyan.
The light falloff you are talking about, could be the effect of less density, allowing a Cyan (not Blue) cast occurring.
Just some food for thought.
Be careful regarding Cyan and Blue casts, they are different and take a while to figure out, just like Magenta and Red casts are different, but sometimes they look the same as well!
One thing you may think about, if you haven't done it yet, is to make a colour ring'a'round of colour casts, this will give you a correct cast for reference.
For example if the cast is bluish and cold looking is it a Cyan or Blue cast? By looking at you own test prints you will quickly ascertain the cast and can then change the filtration with confidence.
Another possible way to alleviate a light fall off when enlarging, is to use a larger lens like a 75/80mm or 105mm and then your 35mm neg will be in a sweet spot of intense light. From my personal experience, light fall off from huge enlargements can sometimes benefit from this procedure, as does a glass holder.
Mick.