What ratio of portland cement do I add to a 50/50 mix of masonry sand & aggregate (pea gravel) ?

1-2-3 will give you a good , strong mix. I've used it many times. The other translates into 1-2.5-2.5, which for all practical purposes is the same. You don't need to be that exact since this really isn't rocket science ! Don't make it too soupy as this weakens the concrete.

If you just use cement, once dried it will be that fragile you come put your finger through it, use sand ether 3 to 1 or 2 to 1, ideal mix that 3 parts sand to one part cement, if you want maximum strength mix concrete which is three parts chips, two parts sand and one part cement.

3 parts cement to 1 part 50/50

I would stick with the ratio just to make sure that your cement is strong enough for every day use. If you don't make it strong enough then you are going to have to worry about erosion and all other factors to it being to soft and falling apart.

the more sand you put in the mix the weaker the mix, just use the regular formula

or you will end up with a crumbling foundation

20% portland

80% 50/50

This will give you a nice foundation that will take moisture well

the doctors advice sounds good... just make sure to use concrete sand and not masons sand...

As long as there's not too much gravel, that should work.