I grind my own pigments (from raw soot in general). With the grief it gives me, I can highly recommend using pigments from a tube! I also size and sometimes make my own paper. Time spent using good quality pigment from a tube and transferring to a well behaved substrate (such as fixed out photo paper) will allow you to perfect your technique; if your technique is not good then using difficult pigments or sizing/making your own paper will cause no-end of failures. As there are so many ways to mess up, gaining a good understanding of how all the variables interact is really important. I find one of the greatest helps to getting consistent results is to keep really detailed notes on my experiments (i.e. quantities, times, temperatures, humidities etc.)
If you do decide to use dry pigments, take time to disperse them well into a liquid before adding the gelatine glop. I ball-mill my soot to get it really fine first, then grind it in a pestle and mortar with a few drops of warm gelatine glop until I get a thick goo. I then add more glop a few drops at a time and keep grinding etc until the soot is all dispersed well. I then pass it through a fine filter to remove any lumps that may remain. Be aware that some pigments do not work well with 'modern' glop recipes (soot is a good example of a difficult pigment); I am using an old soap-based glop formula (real soap, not detergent) that works really well with soot as without the soap it misbehaves. Interestingly many of the tube-based pigments do not work well in the soap-based glop.
The quantities of dry pigment vary depending on what the pigment is and how finely it is ground. My grinding can be considered very coarse compared to tube paint so my pigment load varies from 20g/litre to 100g of dry soot per litre of glop (no, I have not added a zero by mistake!). With the really coarse grinds and at 100g/litre, the tissue is high contrast and has little relief, but feels a bit like sandpaper.
There are no rules really, do not be afraid to experiment (but of course let us know what you find out!).