The first problem is that printing paper is designed to do a rather different job than film so it tends to behave differently when being used as a 'film substitute'.
Firstly, manufacturers do not bother with working out the ISO of printing paper. It's a bit of a moveable feast anyway as it depends upon a variety of factors.
The slowest film I have come across is Lith film at 5 ISO and paper - under average lighting conditions - seems to be a little slower. Taking a starting point somewhere between 2 and 4 ISO would seem reasonable.
Secondly, the colour response of paper is different to film. Graded paper is mostly blue sensitive - once you get to green it records much less, and longer wavelengths don't at all. They are probably more sensitive down into the UV as well.
Multigrade will respond to colour in a slightly different way as it contains a mixture of two emulsions - one sensitive to blue, the other to green.
I would imagine from this (and this is conjecture) that the colour temperature of your light source may have some effect on exposure. Daylight (or lights at 50,000K or above) would give the best results. I wouldn't have thought, though, that colour temp would have much effect until it gets below 3000K - considering that most enlargers use light bulbs running not much above this.
Third, paper has a paper base (well, d'uh!) and this is white so light will be reflected from this during exposure. Any light-coloured or reflective surfaces inside the camera will bounce this back and possibly cause fogging of one kind or another.
Fourthly, paper uses almost the entire exposure range of the emulsion from Dmax to Dmin so there is no 'latitude' like you get in film.
Moving on to exposure:
The biggest problem is hysteresis.
This is what causes reciprocity failure at low levels of exposure.
Any system is reluctant to change and resists it until the energy being put into the system is higher than the energy of resistance.
If you have every tried to push a car on a flat surface you will have come across this. Getting the car moving is the hard part - once you have it moving it seems easier to push.
Emulsion works in a similar way. You need to put enough energy (light) in to the system to start the process of exposure. If the initial energy level is insufficient then no exposure will take place - no matter how long you expose it for.
It's like trying to move the car by pushing gently. You can lean against it for as long as you like but it won't move.
As the ISO of the paper is low it needs far more energy than film to get things going. From this it can be seen that you need to do two things - illuminate your subject as brightly as possible, and use the widest aperture you can.
If you are using a large format camera with a darkslide you can use the darkslide to make a test strip. Pull the slide out in measured increments, doubling the exposure each time. Just like printing. This is by far the best way to assess exposure.
As to the rest I'm afraid you are pretty much on your own. Little or no work has been done on printing paper as film so everything will be trial and error.
I have always used paper as the neg in my pinhole cameras and - by bearing the above limitations and points in mind - I have generally managed to get useable negs.
But paper does have quite severe limits.
) graph on it... unless you coated it yourself? 
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