ok so I would like t first say congrats on your move towards creating a cube. I my self have made 3 and am working on my 4th. so I hope that can at least point you away from making any mistakes that I did in my early cubes. from your post it sounds like your wanting to draft the rtr block over and over again. so for theme (block)cubes I sudgest just taking 1 copy of each card from the block (so in your case about 500ish cards) and poof there ya go. that will support about 12 people an provide a great deal of variance with your cube. That really is the easiest and probably best way to do it. other wise you start to become more regulated in how your cube drafts if you try break packs by rarity.
so other than that here are some few tips that ive learned over the years of doing cubes.
1. sleeve your cube. if your going to take the time to make one and maintain it then sleeve it as well.
2. sleeve and pack lands. if your cube is sleeved then you need lands to play with. I sudgest for a 12 man cube about 65 of each land. you could probably cut it to 50. but I have ran out of lands with 65 of each.
3. take the time to shuffle it. nothing sucks more than getting the pack of 15 commons. when you first draft with it I sudgest this. sit every one at a table. split the cube by color (w,u,b,r,g,gold,split,artifact,lands,) plainswalkers go with their color. give each person a color and tell them to shuffle the pile. shuffle the nonbasic land and artifacts together and keep them separate. then with the other colors as people shuffle have them take half of their color and give it to another person. keep doing that until it is shuffled. I do this at the beginning of every session. it takes about 15 min but well worth it.
4. learn to deal your cube out. so after shuffling you should have two piles. the artifact/land and the everything else. for every pack I do 3 artifact land and 12 others. this means everyone has even shot at mana fixing which is crucial. I then lay thepacks out on the table and allow everyone to pick any3 they like (don't just simply give em to people. people like choices in the matter).
5. separate the basic land, nonbasic land and artifact out after you are done playing.
6. have fun
I hope this was of some help